One in the past a villain (4)
I believe the answer is:
iago
'a villain' is the definition.
(I've seen this before)
'one in the past' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
'i'+'ago'='IAGO'
(Other definitions for iago that I've seen before include "Shakespeare's jealous villain" , "villain in play" , "Othello's evil counsellor" , "Othello's treacherous counsellor" , "Othello's nemesis" .)