Over the hill (4,2)
I believe the answer is:
past it
'over the hill' is the definition.
(both can mean getting old)
This is the entire clue.
(Other definitions for past it that I've seen before include "No longer capable" , "not helped by age" , "No longer in one's prime" , "Informally too old to function" , "No longer able, too old or decrepit" .)