Unique sound of Motown on the radio (4)
I believe the answer is:
sole
'unique' is the definition.
('sole' can be a synonym of 'unique')
'sound of motown on the radio' is the wordplay.
'sound of motown' becomes 'soul' (Motown is record label known for soul music).
'on the radio' indicates a 'sounds like' (homophone) clue.
'soul' is a homophone of 'SOLE'.
(Other definitions for sole that I've seen before include "Fish; part of foot" , "Fish - alone" , "Shoe part -- lone" , "Underside of footwear" , "European flatfish" .)