'After dark' piece on the radio (5)
I believe the answer is:
night
'after dark' is the definition.
'piece on the radio' is the wordplay.
'piece' becomes 'knight' (knight is a kind of piece).
'on the radio' shows a homophone (sound like).
'knight' sounds like 'NIGHT'.
(Other definitions for night that I've seen before include "Time for bed" , "death" , "Dark hours" , "For most, the time for sleep" , "The dark time" .)