Legal document about poor service that’s cancelled (7)
I believe the answer is:
deleted
'cancelled' is the definition.
('delete' can be a synonym of 'cancel')
'legal document about poor service' is the wordplay.
'legal document' becomes 'deed' (deed is a kind of legal document).
'about' indicates putting letters inside.
'poor service' becomes 'let' (tennis).
'deed' going around 'let' is 'DELETED'.
'that's' acts as a link.
(Other definitions for deleted that I've seen before include "Struck out" , "Effaced" , "Erased" , "Removed" , "Expunged" .)