Small bird seen by old man over the hill (4,2)
I believe the answer is:
past it
'over the hill' is the definition.
(both can mean getting old)
'small bird seen by old man' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'tit' (tit is a kind of bird).
'seen by' is a charade indicator (letters next to each other).
'old man' becomes 'pa' (both can mean father).
's'+'tit'='stit'
'stit' after 'pa' is 'PAST IT'.
(Other definitions for past it that I've seen before include "No longer competent" , "Far from fresh" , "Too old for anything" , "not helped by age" , "No longer able, too old or decrepit" .)