The figure in the poster is slim (7)
I believe the answer is:
slender
'is slim' is the definition.
(synonyms)
'the figure in the poster' is the wordplay.
'the figure' becomes 'l' (I've seen this in other clues).
'in' indicates putting letters inside.
'the poster' becomes 'sender' (somebody sending or posting something).
'l' going within 'sender' is 'SLENDER'.
(Other definitions for slender that I've seen before include "Willowy" , "Slight, slim" , "Shakespearean character" , "remote" , "Meagre" .)