A model problem? (5)

I believe the answer is:

poser

I believe this clue is a double definition.

'a 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 "Maybe model" , "Tricky question" , "Puzzle using ropes" , "Particularly baffling problem" , "Hard question" .)

I've seen this clue in the Evening Standard.

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