Line to appear on the radio (4)
I believe the answer is:
seam
'line' is the definition.
(I know that line can be written as seam)
'appear on the radio' is the wordplay.
'appear' becomes 'seem' (synonyms).
'on the radio' indicates a 'sounds like' (homophone) clue.
'seem' sounds like 'SEAM'.
'to' acts as a link.
(Other definitions for seam that I've seen before include "A stitched join in a garment" , "Sewn join" , "Joint between two edges" , "Stratum (of a mineral)" , "Join - stratum of ore" .)