Difficult question for a model (5)
I believe the answer is:
poser
I believe this clue is a double definition.
'difficult question' is the first definition.
(I've seen this before)
'for a model' is the second definition.
('poser' can be a synonym of 'model')
(Other definitions for poser that I've seen before include "Hard question" , "Particularly baffling problem" , "Conundrum - model?" , "One pretends to be something they are not" , "Problem set by the model" .)