Account for former partner getting even (7)
I believe the answer is:
explain
'account for' is the definition.
(thesaurus)
'former partner getting even' is the wordplay.
'former partner' becomes 'ex'.
'getting' says to put letters next to each other.
'even' becomes 'plain' ('plain' could historically mean smooth and even).
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Interpret" , "Elucidate, make clear" , "Give reasons for" , "Describe" , "Make intelligible and account for" .)