Managed to bring about endless resentment (7)
I believe the answer is:
rancour
'resentment' is the definition.
(synonyms)
'managed to bring about endless' is the wordplay.
'managed' becomes 'ran' (I've seen this in another clue).
'to bring about' becomes 'court' (as in courting controversy).
'endless' means to remove the last letter.
'court' with its final letter taken off is 'cour'.
'ran'+'cour'='RANCOUR'
(Other definitions for rancour that I've seen before include "Spite" , "Long-lasting bitterness" , "Bitter feeling or ill will" , "Deep-seated bitterness" , "Feeling of bitterness or dislike" .)