Singer's altered a lot (4)
I believe the answer is:
alto
'singer's' is the definition.
(alto is a kind of singer)
'altered a lot' is the wordplay.
'altered' is an anagram indicator.
'a'+'lot'='alot'
'alot' anagrammed gives 'ALTO'.
(Other definitions for alto that I've seen before include "someone to sing" , "Kind of singing voice, male or female" , "Highest male voice: lowest female voice" , "Woman's singing voice" , "Singing voice between tenor and soprano" .)