Against Dickensian villain being beheaded (4)
I believe the answer is:
agin
'against' is the definition.
(dialect form of against)
'dickensian villain being beheaded' is the wordplay.
'dickensian villain' becomes 'fagin' (I've seen this before).
'being beheaded' means to remove the first letter.
'fagin' with its initial letter removed is 'AGIN'.
(Other definitions for agin that I've seen before include "Not a fan of" , "Against (facetious)" , "Hostile to" , "Not in favour of" , "Dialect form of against" .)