One past Shakespearean villain (4)
I believe the answer is:
iago
'shakespearean villain' is the definition.
(I've seen this before)
'one past' is the wordplay.
'one' becomes 'i' (Roman numeral).
'past' becomes 'ago' ('ago' can be similar in meaning to 'past').
'i'+'ago'='IAGO'
(Other definitions for iago that I've seen before include "Ancient role" , "stage villain" , "villain in play" , "Othello's Ancient" , "Shakespearean villain" .)