Model problem (5)
I believe the answer is:
poser
I believe this clue is a double definition.
'model' is the first definition.
('poser' can be a synonym of 'model')
'problem' is the second definition.
(poser is a kind of problem)
(Other definitions for poser that I've seen before include "Tricky problem" , "one who has airs and graces" , "Sticky problem" , "Tricky clue" , "Conundrum - model?" .)