Leave it to a former partner (4)
I believe the answer is:
exit
'leave' is the definition.
(synonyms)
'it to a former partner' is the wordplay.
'to a' means one lot of letters go next to another (I've seen this in other clues).
'former partner' becomes 'ex' (an ex-partner).
'it' after 'ex' is 'EXIT'.
(Other definitions for exit that I've seen before include "Way to leave" , "Way to go!" , "Road leaving motorway" , "Leave the building" , "Actor's departure" .)