Forgotten, the old man on street (4)
I believe the answer is:
past
'forgotten' is the definition.
(thesaurus)
'the old man on street' is the wordplay.
'the old man' becomes 'pa' ('pa' can be a synonym of 'old man').
'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 "Bygone" , "The time that has elapsed" , "tense" , "Finished, gone" , "historical times" .)