Old man having part to play in prisoner's conditional release (6)
I believe the answer is:
parole
'prisoner's conditional release' is the definition.
'old man having part to play' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'having' says to put letters next to each other.
'part to play' becomes 'role' (I've seen this before).
'pa'+'role'='PAROLE'
'in' is the link.
(Other definitions for parole that I've seen before include "Release of prisoner promising good behaviour" , "Early release on promise of good behaviour" , "(One's) word" , "Release (a prisoner) on the promise of good conduct" , "Temporary release (of a prisoner)" .)