Never returned notes on vengeance (7)
I believe the answer is:
revenge
'vengeance' is the definition.
(thesaurus)
'never returned notes on' is the wordplay.
'returned' shows that the letters should be reversed in order.
'notes' becomes 'ge' ().
'on' means one lot of letters go next to another.
'never' reversed gives 'reven'.
'reven'+'ge'='REVENGE'
(Other definitions for revenge that I've seen before include "Ship remembered in verse" , "An eye for an eye" , "Retribution" , "Satisfaction taken for injury" , "Malicious retaliation" .)