Question model (5)
I believe the answer is:
poser
I believe this is a double definition.
'question' is the first definition.
(poser is a kind of question)
'model' is the second definition.
('poser' can be a synonym of 'model')
(Other definitions for poser that I've seen before include "Problem set by the model" , "Tricky question" , "A puzzle made of ropes" , "Maybe model" , "Hard problem" .)