Account for former partner left in agony (7)
I believe the answer is:
explain
'account for' is the definition.
(thesaurus)
'former partner left in agony' is the wordplay.
'former partner' becomes 'ex'.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'agony' becomes 'pain' (I've seen this before).
'l' put into 'pain' is 'plain'.
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Describe" , "Interpret" , "Give reasons for" , "Show the whys and wherefores" , "Elucidate, clarify" .)