Promise dad part in play (6)
I believe the answer is:
parole
'promise' is the definition.
(parole is a kind of promise)
'dad part in play' is the wordplay.
'dad' becomes 'pa' ('pa' can be a synonym of 'dad').
'part in play' becomes 'role' (I've seen this before).
'pa'+'role'='PAROLE'
(Other definitions for parole that I've seen before include "(One's) word" , "Conditionally release" , "Release of prisoner promising good behaviour" , "Provisional release (for prisoner)" , "Promise (that prisoner is released on)" .)