Second-highest voice in a four-part chorus (4)
I believe the answer is:
alto
'second-highest voice in a four-part chorus' 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 "Singing range" , "Singing voice between tenor and soprano" , "Lowest female singing voice" , "Unisex vocal range" , "The highest adult male singing voice" .)