Leave former partner with the thing (4)
I believe the answer is:
exit
'leave' is the definition.
('exit' can be a synonym of 'leave')
'former partner with the thing' is the wordplay.
'former partner' becomes 'ex'.
'with' means one lot of letters go next to another.
'the thing' becomes 'it'.
'ex'+'it'='EXIT'
(Other definitions for exit that I've seen before include "Sudden rush of pleasure" , "Withdrawal" , "Move out" , "Escape route?" , "Means of egress" .)