Voice not pure on the radio (4)
I believe the answer is:
bass
'voice' is the definition.
(I know that bass is a type of voice)
'not pure on the radio' is the wordplay.
'not pure' becomes 'base' (base can mean impure or adulterated).
'on the radio' indicates a 'sounds like' (homophone) clue.
'base' sounds like 'BASS'.
(Other definitions for bass that I've seen before include "Variety of sea and freshwater perch" , "Performer" , "Sea fish; voice" , "Male singer and saltwater fish" , "Freshwater fish and lowest voice" .)