Aggravate former partner over account and berate in error (10)
I believe the answer is:
exacerbate
'aggravate' is the definition.
('exacerbate' can be a synonym of 'aggravate')
'former partner over account and berate in error' is the wordplay.
'former partner' becomes 'ex'.
'over' is a charade indicator (letters next to each other).
'account' becomes 'ac'.
'and' means one lot of letters go next to another.
'in error' indicates anagramming the letters.
'berate' is an anagram of 'erbate'.
'ex'+'ac'+'erbate'='EXACERBATE'
(Other definitions for exacerbate that I've seen before include "Make (a bad thing) worse" , "Worsen" , "Provoke" .)