Improved on G&S ditty (4)
I believe the answer is:
song
'ditty' is the definition.
(I've seen this before)
'improved on gs' is the wordplay.
'improved' is an anagram indicator.
'on'+'gs'='ongs'
'ongs' is an anagram of 'SONG'.
(Other definitions for song that I've seen before include "Unnecessary fuss" , "Piece of vocal music" , "Carol, for example" , "You work best when on this" , "Musical rendition" .)