Barbershop quartet part (4)
I believe the answer is:
bass
'barbershop quartet part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bass that I've seen before include "Booming percussion instrument" , "Low-pitched; a fish" , "Large fish - singer" , "bottom of sound?" , "Fish - low voice" .)