Slim student in poster (7)
I believe the answer is:
slender
'slim' is the definition.
(synonyms)
'student in poster' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'in' is an insertion indicator.
'poster' becomes 'sender' (I've seen this before).
'l' put inside 'sender' is 'SLENDER'.
(Other definitions for slender that I've seen before include "Fine" , "Feeble" , "Slight, slim" , "Meagre" , "with a narrow outlook" .)