Narrow lake in poster (7)
I believe the answer is:
slender
'narrow' is the definition.
(similar in meaning)
'lake in poster' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'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 "Barely sufficient (means)" , "Feeble" , "Barely sufficient; thin" , "Gracefully slim" , "remote" .)