Promise father a part (6)
I believe the answer is:
parole
'promise' is the definition.
(parole is a kind of promise)
'father a part' is the wordplay.
'father' becomes 'pa' (pa is a kind of father).
'a part' becomes 'role' ('role' can be a synonym of 'part'**).
'pa'+'role'='PAROLE'
(Other definitions for parole that I've seen before include "limited release" , "Early release on promise of good behaviour" , "Early conditional release for prisoner in jail" , "Temporary release (of a prisoner)" , "Promise of not escaping etc" .)