Singer who sounds low... (4)
I believe the answer is:
bass
'singer' is the definition.
(thesaurus)
'sounds low' is the wordplay.
'sounds' shows a homophone (sound like).
'low' becomes 'base' ('base' can be a synonym of 'low').
'base' is a homophone of 'BASS'.
'who' acts as a link.
(Other definitions for bass that I've seen before include "A fish, or the lowest pitch in music" , "bottom of sound?" , "Performer" , "Low fellow" , "part of score" .)