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 "Puzzle using ropes", "Maybe model", "What's difficult", "show-off", "Ropes in with puzzle", "One pretends to be something they are not", "A problem", "Show-off", "Question; one trying to impress" and "Tricky clue".)