Difficult question for model (5)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'difficult question' is the first definition.
(I've seen this before)
'model' is the second definition.
('poser' can be a synonym of 'model')
'for' is the link.
(Other definitions for poser that I've seen before include "Attitudiniser", "Toughie", "Question; one trying to impress", "Problem set by the model", "Someone behaving so as to be noticed", "Hard question", "Tricky problem", "Conundrum - model?", "Brain-teaser" and "Baffling question or problem".)