Partner's second to leave girl (4)
I believe the answer is:
ally
'girl' is the definition.
'partner's second to leave' is the wordplay.
'partner's' becomes 'allys' (I've seen this in other clues).
'second' becomes 's' (common abbreviation).
'to leave' suggests deleting specific letters.
'allys' with 's' taken out is 'ALLY'.
(Other definitions for ally that I've seen before include "Colleague, confederate" , "Supporter in a conflict" , "Team up" , "Fighting partner" , "Associate that provides assistance" .)