Father's bird is over the hill (4,2)
I believe the answer is:
'over the hill' is the definition.
(both can mean getting old)
'father's bird' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'bird' becomes 'tit' (tit is a kind of bird).
'is' is the link.
(Other definitions for past it that I've seen before include "Unable to function any more, perhaps due to age" , "undrinkable?" , "Far from fresh" , "Spit at, being too old or decrepit" , "Informally too old to function" .)