Leave former partner with it (4)
I believe the answer is:
exit
'leave' is the definition.
(synonyms)
'former partner with it' is the wordplay.
'former partner' becomes 'ex' (an ex-partner).
'with' says to put letters next to each other.
'ex'+'it'='EXIT'
(Other definitions for exit that I've seen before include "Leave the building" , "Withdrawal" , "Way out sign" , "Means of egress" , "Road leaving motorway" .)