Ambassador forgets old resentment (4)
I believe the answer is:
envy
'resentment' is the definition.
(envy is a kind of resentment)
'ambassador forgets old' is the wordplay.
'ambassador' becomes 'envoy' (I've seen this in another clue).
'forgets' indicates named letters should be taken away.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'envoy' with 'o' taken away is 'ENVY'.
(Other definitions for envy that I've seen before include "Jealousy, begrudgery" , "Feeling of discontent" , "grudging discontent" , "Jealous emotion - deadly sin" , "Wish for thing or quality belonging to another" .)