Can't speak specifically to clowns as I haven't kept a lot of them, but IME it is not uncommon at all for a new fish to remain hidden for a day or even two. The last wrasse I introduced to my aquarium disappeared for a week before he started showing up. That said, it could also have died. It is hard to say what might have happened. Some more information, like how you acclimated, temp, water parameters, etc. might help us make a more educated guess.
"Listen to some of these guys talk, and it's like they were born from their momma's belly with a fishkeeping encyclopedia in one hand and an API kit in the other" (unrevealed).