Thin line in poster? (7)
I believe the answer is:
slender
'thin' is the definition.
(I know that thin can be written as slender)
'line in poster?' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' indicates putting letters inside.
'poster?' becomes 'sender' (somebody sending or posting something).
'l' going into 'sender' is 'SLENDER'.
(Other definitions for slender that I've seen before include "Shakespearean character" , "Barely sufficient (means)" , "Thin and graceful" , "Slight in form" , "Slim and supple" .)