A member of the opposition in Roman times (4)
I believe the answer is:
anti
'a member of the opposition' is the definition.
(I've seen this in another clue)
'in roman times' is the wordplay.
'in' says the answer is hidden in the clue.
'ANTI' can be found hidden inside 'roman times'.
(Other definitions for anti that I've seen before include "Policy's opponent" , "Who pooh-poohs all" , "Opposition member" , "Negative vote" , "One who is against (something)" .)