Melody on radio extended by and by (7)
I believe the answer is:
erelong
'by and by' is the definition.
'melody on radio extended' is the wordplay.
'melody' becomes 'air' (air can mean a tune or melody).
'on radio' shows a homophone (sound like).
'extended' becomes 'long' (extended can mean longer than usual).
'air' is a homophone of 'ere'.
'ere'+'long'='ERELONG'
(Other definitions for erelong that I've seen before include "Soon (archaic)" , "Soon (poetic)" , "Soon (poetically)" , "Soon (arch.)" .)