Father will take part - it's a promise (6)
I believe the answer is:
parole
'it's a promise' is the definition.
(parole is a kind of promise)
'father will take part' is the wordplay.
'father' becomes 'pa' (pa can mean dad or father).
'will take' is a charade indicator (letters next to each other).
'part' becomes 'role' (eg a theatrical role).
'pa'+'role'='PAROLE'
(Other definitions for parole that I've seen before include "Lop ear (anag.)" , "Temporary release (of a prisoner)" , "Word - condition of liberty" , "Release (a prisoner) on the promise of good conduct" , "Promise - conditional release" .)