Father leaves the pair of them (5)
I believe the answer is:
their
'of them' is the definition.
(I've seen this before)
'father leaves the pair' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'leaves' suggests deleting specific letters.
'the'+'pair'='thepair'
'thepair' with 'pa' taken away is 'THEIR'.
(Other definitions for their that I've seen before include "belonging to an individual" , "Belonging to him (perhaps wrongly)" , "Some would say his (or her)" , "Belonging to those people" , "Relating to them" .)