Thin line into transmitter (7)
I believe the answer is:
slender
'thin' is the definition.
('slender' can be a synonym of 'thin')
'line into transmitter' is the wordplay.
'line' becomes 'l'.
'into' is an insertion indicator.
'transmitter' becomes 'sender' ('sender' can be a synonym of 'transmitter').
'l' put within 'sender' is 'SLENDER'.
(Other definitions for slender that I've seen before include "Spare" , "Thin and graceful" , "Slim and supple" , "Svelte" , "inadequate" .)