One who's past it (3-4)
I believe the answer is:
has-been
'one who's past it' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for has-been that I've seen before include "Banshee (anag.)" , "One who's past it" , "Person no longer famous or successful" , "One no longer important" , "One-time leading light who's faded" .)