Head tenor and alto performed full amount (5)
I believe the answer is:
total
'full amount' is the definition.
(I've seen this before)
'head tenor and alto performed' is the wordplay.
'head' indicates taking the first letters.
'and' means one lot of letters go next to another.
'performed' indicates an anagram.
The first letter of 'tenor' is 't'.
'alto' is an anagram of 'otal'.
't'+'otal'='TOTAL'
(Other definitions for total that I've seen before include "Complete, entire amount" , "Final tally" , "Whole, complete" , "Complete sum" , "Not qualified" .)