The "A" in SATB (4)
I believe the answer is:
alto
'the a in satb' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alto that I've seen before include "Voice part" , "Second of four parts (in singing)" , "Unisex vocal range" , "Singing voice between tenor and soprano" , "Member of choir" .)