Only one type of music on the radio (4)
I believe the answer is:
sole
'only one' is the definition.
'type of music on the radio' is the wordplay.
'type of music' becomes 'soul' (I've seen this before).
'on the radio' indicates a 'sounds like' (homophone) clue.
'soul' sounds like 'SOLE'.
(Other definitions for sole that I've seen before include "Fish - alone" , "One and only; part of foot" , "A fish; part of shoe" , "Kind of fish - lose it" , "Fish; part of foot" .)