Picture the setter's maturity (5)
I believe the answer is:
image
'picture' is the definition.
('image' can be a synonym of 'picture')
'the setter's maturity' is the wordplay.
'the setter's' becomes 'im' (I've seen this in other clues).
'maturity' becomes 'age' (I've seen this before).
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Mental or physical representation" , "Epitome" , "Likeness of person or thing" , "Simile, metaphor" , "Public persona" .)