On May 20, he was attached to the 4th Canadian Infantry Brigade, providing communications for The Royal Regiment of Canada, a Toronto unit, The Royal Hamilton Light Infantry, The Essex Scottish Regiment from Windsor and Ottawa's Cameron Highlanders which was a machine gun unit. In late August Clare embarked for Britain, arriving in Gourock, Scotland on September 5. He was late arriving at the base at Aldershot, England and was confined to barracks for two days. Training and war games continued and Clare attended courses to improve his skills. By mid-April, 1941, he qualified as a lineman and was promoted to acting lance corporal. His job was to run telephone wires between infantry and headquarters.

From February 15 to March 28, 1942, Clare attended a non-commissioned officers' course for working in headquarters. Around this time, his father moved the family to his new shop at 1250 Woodbine Avenue, on the corner of Woodbine and Dunkirk Road. They lived above the store. Clare's brother George was working in the store as a butcher.

In 1942, the Allied high command thought that a raid on a French port, to hold it for a few days, destroying enemy defences and then withdrawing would boost Allied morale and show the Soviet Union, who was fighting on the Eastern Front, that there was some commitment to engage the enemy on the Western Front. Canadian units were chosen for the task as they had had almost no battle experience to date. It was initially called Operation RUTTER, to be launched in early July with three months of amphibious training taking place along the south coast of England leading up to the operation. Poor weather caused the mission to be cancelled at the last minute.

Next, Operation JUBILEE was a resurrection of Operation RUTTER, to be launched in the middle of August. The target was Dieppe, on the French coast. The 4th Canadian Infantry Brigade was among those selected for the operation and the command headquarters was to remain afloat on the destroyers HMS Calpe and HMS Fernie. Clare was stationed on one of the two ships, one of fourteen signalmen in his division. The signals plan had been ongoing since May 17 and there had been three full-scale exercises to try it out leading up to Operation RUTTER.

In the early morning of August 19, 1942, over 6,000 men, mostly Canadians, stormed the Dieppe beach to disastrous results. Some beaches were protected by seawalls and left the Canadians as sitting ducks from the guns of the enemy above them. The enemy had had some advance warning and were prepared for the invasion, sending out Luftwaffe planes to bomb the beaches and the supporting ships. Communication failed with the shore regiments as almost all the radio operators who landed were killed and their transmitters destroyed by enemy fire. HMS Calpe acted as a hospital ship, caring for 278 casualties and she lost nearly a quarter of her crew to casualties. HMS Fernie was subjected to heavy air attacks. Of the Canadians who landed, the casualty rate was 68 percent. By early afternoon, the Allies were in retreat. The Dieppe Raid has been recognized as a colossal blunder, a textbook example of what not to do in battle. Lord Mountbatten, who had overseen the plan, immediately tried to put a positive spin on the fiasco by saying that the lessons learned would save countless lives during the upcoming European invasion. That was cold comfort to the 916 Canadian families who lost sons or husbands and the 1,946 Canadians who were taken prisoner.

Clare returned to England, no doubt shaken by the experience. As a Christmas present, he received a promotion to Lance Corporal. On January 5, 1943, he wrote a letter to his family telling them that he had been ill on New Year's Day.  On January 29 Clare began experiencing severe pain in his stomach, although he hadn't eaten anything out of the ordinary. He was seen by the medical orderly and returned to his billet, having been given some stomach powder. During the night his stomach began bothering him again, but it subsided. The next morning the medical orderly suspected appendicitis and recommended taking Clare to the hospital. Clare told his lieutenant that he felt fairly well. The lieutenant had known Clare for five months and knew that Clare had never been on sick parade for stomach pain before. At 12:30 pm he was taken in an ambulance accompanied by the medical orderly on the 30 kilometre journey to the Canadian Military Hospital in Horsham. On leaving the base, he told the lieutenant that he was still quite comfortable.

When he arrived at the hospital at 2 pm, the doctors diagnosed acute appendicitis and Clare was operated on at 4 pm. He died on the operating table shortly after 6 pm. During the operation, the surgeon noticed that he must have had several previous attacks of appendicitis. The post mortem concluded that he had an enlarged thymus, plaque in his arteries and, along with being overweight, this made him a poor anaesthetic risk.