This singer is a lot improved (4)
I believe the answer is:
alto
'this singer' is the definition.
(type of singing voice)
'a lot improved' is the wordplay.
'improved' indicates anagramming the letters.
'lot' with letters rearranged gives 'lto'.
'a'+'lto'='ALTO'
'is' is the link.
(Other definitions for alto that I've seen before include "Singing range" , "Singer's hidden inside chemical toilet" , "The highest adult male singing voice" , "Voice part" , "Singing voice between tenor and soprano" .)