Dad’s time — history! (4)
I believe the answer is:
past
'history' is the definition.
(I've seen this before)
'dad's time' is the wordplay.
'dad's' becomes 'pas' ('pa' can be a synonym of 'dad').
'time' becomes 't' (abbreviation).
'pas'+'t'='PAST'
(Other definitions for past that I've seen before include "over and done with" , "Yesteryear" , "Ended" , "Days gone by" , "Bygone" .)