You can clear over anything. We used to spray completes with enamel and then clear it with urethane clear. Heck, we used to even get the color all down and then mix up some 1/2 color 1/2 clear and put a coat or two on, then one coat of all clear. Some of the best paint jobs I've ever seen come out with that sort of combo.
Using a urethane color and an enamel clear though, not 100% sure on that. Sorta like the phrase beer on top of whiskey, sorta risky; and whiskey on top of beer never fear. I probably would not start hammering enamel clear on top of still tacky urethane. I'm just not really well versed in that mixture to be honest. I'll have to go talk to my painter a little, I'm only 40, he's been painting as long as I've been alive.
