Forgotten old man on street (4)
I believe the answer is:
past
'forgotten' is the definition.
(thesaurus)
'old man on street' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'on' is a charade indicator (letters next to each other).
'street' becomes 'st' (**).
'pa'+'st'='PAST'
(Other definitions for past that I've seen before include "A person's earlier life history" , "The time that has elapsed" , "Ended" , "Second visitor in 5 [A CHRISTMAS CAROL]" , "Bygone" .)