Shakespearean villain, one in the past (4)
I believe the answer is:
iago
'shakespearean 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' (synonyms).
'i'+'ago'='IAGO'
(Other definitions for iago that I've seen before include "Advisor to soldier" , "Othello's evil counsellor" , "The villain who tricked Othello" , "stage villain" , "Shakespearean villain" .)