Alto performed, on the whole (5)
I believe the answer is:
total
'whole' is the definition.
(I know that total is a type of whole)
'alto performed on the' is the wordplay.
'performed' indicates an anagram.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'alto' is an anagram of 'toal'.
'toal' enclosing 't' is 'TOTAL'.
(Other definitions for total that I've seen before include "Sum, aggregation" , "Absolute; sum" , "Whole, complete" , "Aggregate, sum" , "Utter" .)