Father's period for diversion (7)
I believe the answer is:
pastime
'for diversion' is the definition.
(pastime is a kind of diversion)
'father's period' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'period' becomes 'time' (period of time).
'pas'+'time'='PASTIME'
(Other definitions for pastime that I've seen before include "doing the crossword, say" , "diversion" , "Hobby; regular activity" , "Leisure pursuit" , "Leisure activity" .)