Model puzzle (5)
I believe the answer is:
poser
This clue is a double definition.
'model' is the first definition.
(I know this)
'puzzle' is the second definition.
(I've seen this in another clue)
(Other definitions for poser that I've seen before include "What's difficult" , "Conundrum - model?" , "Person still being portrayed" , "subject for an artist?" , "Baffling question or problem" .)