Promise the old man a part (6)
I believe the answer is:
parole
'promise' is the definition.
(parole is a kind of promise)
'the old man a part' is the wordplay.
'the old man' becomes 'pa' ('pa' can be a synonym of 'old man').
'a part' becomes 'role' ('role' can be a synonym of 'part'**).
'pa'+'role'='PAROLE'
(Other definitions for parole that I've seen before include "Conditional release from prison" , "Conditionally release" , "Provisional release (for prisoner)" , "Release (a prisoner) on the promise of good conduct" , "Lop ear (anag.)" .)