Four-part harmony part (4)
I believe the answer is:
alto
'four-part harmony part' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for alto that I've seen before include "Key old > male who scores high" , "someone to sing" , "Highest male voice: lowest female voice" , "Kind of singing voice, male or female" , "A singing voice or pitch" .)