Resentment at being interrupted by new lover (7)
I believe the answer is:
gallant
'lover' is the definition.
(I've seen this before)
'resentment at being interrupted by new' is the wordplay.
'resentment' becomes 'gall' (synonyms).
'being interrupted by' is an insertion indicator.
'new' becomes 'n'.
'at' going around 'n' is 'ant'.
'gall'+'ant'='GALLANT'
(Other definitions for gallant that I've seen before include "Brave and chivalrous like knight of old" , "Heroic; chivalrous" , "Dashing gentleman" , "Courtly" , "Dashing, debonair young man" .)