Hobby in old man's day? (7)
I believe the answer is:
pastime
'hobby' is the definition.
(thesaurus)
'old man's day?' is the wordplay.
'old man's' becomes 'pas' (both can mean father).
'day?' becomes 'time' (day is a kind of time).
'pas'+'time'='PASTIME'
'in' is the link.
(Other definitions for pastime that I've seen before include "Leisure activity" , "diversion" , "Leisure pursuit" , "relaxation" , "doing the crossword, say" .)