Register changes a lot (4)
I believe the answer is:
alto
'register' is the definition.
(vocal register)
'changes a lot' is the wordplay.
'changes' indicates an anagram.
'a'+'lot'='alot'
'alot' is an anagram of 'ALTO'.
(Other definitions for alto that I've seen before include "Lowest female singing voice" , "Woman's singing voice" , "Unisex vocal range" , "Voice part" , "The highest adult male singing voice" .)