WHAT DOES A BANK robbery have to do with a bearded Winnipeg clown named Uncle Al? It all depends on your interpretation of this story, but in any event the lady who hired Uncle Al to entertain at her children’s birthday party will know now why he was late for the 4 p.m. date.    Uncle Al is actually Al Simmons, a member of the popular Kornstock musical group in Winnipeg. He moonlights quite frequently as an entertainer for kids’ birthday parties, gets all dressed up in a grotesque, colorful costume, takes along balloons and all the other tricks of the clown’s trade.    Just by coincidence, shortly after a bank robbery last week, Uncle Al, in full costume and gaily-painted make-up on his face that completely disguised him, was heading up McPhillips to the birthday party. Suddenly an unmarked police car cruiser with two female officers in it motioned for him to pull over.    The clown was bewildered, but since the ladies flashed badges he had no reason to doubt their identity. When he pulled over to the side of the street, he soon had another marked cruiser behind him, one in front of him, and a fourth on the opposite side of the street.    By this time Uncle Al was sweating profusely through his make-up and completely in the dark as to the reason the four carloads of officers were hemming him in. "Look, I’m a clown, I got balloons to prove it," he said with shaking hands as he pulled out a few of the gadgets from his clown bag.    When the officers finally decided he was really a clown, they waved him on but for a while he was too nervous to drive. So he phoned his manager Len Andree to tell him the story and asked that he call the birthday party to explain that he would be late. Len, who had already been phoned by the distraught mother who wanted to know what happened to the clown, said he didn’t think she believed that Uncle Al was late because he "might have been mistaken for a bank robber in disguise."    It wasn’t one of the clown’s funniest days; he’s laughing about it now, but he said he’s still a little embarrassed by the whole incident.