One in opposition essential to Victorian times (4)
I believe the answer is:
anti
'one in opposition' is the definition.
(I've seen this in another clue)
'essential to victorian times' is the wordplay.
'essential to' indicates a hidden word (letters in the inside or essence of the clue).
'ANTI' can be found hidden inside 'victorian times'.
(Other definitions for anti that I've seen before include "Prefix meaning against" , "one opposing?" , "Against, informally" , "Someone opposed (to a policy?)" , "Hostile" .)