One violently against right-winger (6)
I believe the answer is:
neocon
'right-winger' is the definition.
(I've seen this before)
'one violently against' is the wordplay.
'violently' indicates an anagram.
'against' becomes 'con' (as in pros and cons, for and against).
'one' anagrammed gives 'neo'.
'neo'+'con'='NEOCON'
(Other definitions for neocon that I've seen before include "A supporter of the policies of President George Bush? (abbr)" , "Right-wing American" , "politician on the right" , "Right-winger" , "A right winger" .)