Single was successful on the radio (3)
I believe the answer is:
one
'single' is the definition.
(a single thing - only one)
'was successful on the radio' is the wordplay.
'was successful' becomes 'won' (I've seen this in another clue).
'on the radio' indicates a 'sounds like' (homophone) clue.
'won' is a homophone of 'ONE'.
(Other definitions for one that I've seen before include "Single, sole" , "Hour after midnight" , "Unity" , "Single person or thing" , "Single, unitary" .)