Thin line in transmitter (7)
I believe the answer is:
slender
'thin' is the definition.
(I know that thin can be written as slender)
'line in transmitter' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' is an insertion indicator.
'transmitter' becomes 'sender' (synonyms).
'l' put into 'sender' is 'SLENDER'.
(Other definitions for slender that I've seen before include "Gracefully thin and elegant" , "Ann Page's suitor" , "Fine" , "Meagre" , "with a narrow outlook" .)