Laugh when the singer changes key in it (7)
I believe the answer is:
snigger
'laugh' is the definition.
(snigger is a kind of laugh)
'the singer changes key in it' is the wordplay.
'changes' indicates anagramming the letters.
'key' becomes 'g' (I've seen this in other clues**).
'in it' is an insertion indicator.
'singer' is an anagram of 'sniger'.
'sniger' going around 'g' is 'SNIGGER'.
'when' acts as a link.
(Other definitions for snigger that I've seen before include "Laugh in a scornful way" , "Giggle" , "Titter" , "Surreptitious laugh" , "Dirty laugh" .)