Large in poster, and slim (7)
I believe the answer is:
slender
'slim' is the definition.
(synonyms)
'large in poster' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'in' is an insertion indicator.
'poster' becomes 'sender' (somebody sending or posting something).
'l' going inside 'sender' is 'SLENDER'.
'and' acts as a link.
(Other definitions for slender that I've seen before include "Lean" , "Meagre" , "short of a pound or two of flesh?" , "Shakespearean character" , "Svelte" .)