Old dad’s a singer (4,2)
I believe the answer is:
past it
'old' is the definition.
I know nothing about this answer so I can't tell whether this works.
'dad's a singer' is the wordplay.
'dad's' becomes 'pa's' (both can mean one's father).
'a singer' becomes 'tit' (type of songbird).
'pas'+'tit'='PAST IT'
Can you help me to learn more?
(Other definitions for past it that I've seen before include "Informally too old to function" , "Getting over the hill" , "No longer able, too old or decrepit" , "undrinkable?" , "not helped by age" .)