Laugh when the singer goes off key in it (7)
I believe the answer is:
snigger
'laugh' is the definition.
(snigger is a kind of laugh)
'the singer goes off key in it' is the wordplay.
'goes off' indicates an anagram.
'key' becomes 'g' (musical key).
'in it' indicates putting letters inside.
'singer' is an anagram of 'sniger'.
'sniger' going around 'g' is 'SNIGGER'.
'when' is the link.
(Other definitions for snigger that I've seen before include "Sly, stifled laugh" , "Surreptitious laugh" , "Half-suppressed laugh" , "Suppressed laugh" , "Laugh disrespectfully" .)