Crossword Genius

Fanatic regularly struck opponent (4)

Ross

I believe the answer is:

anti

'opponent' is the definition.
(someone opposed to something)

I don't understand how the rest of the clue works.

(Other definitions for anti that I've seen before include "An opponent - opposed to party policy, etc" , "Definitely not pro" , "Naysayer" , "one opposing?" , "Hostile" .)

I've seen this clue in The Guardian.
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