Harm unknown anti-capitalist (4)

I believe the answer is:

marx

I'm a little stuck... Click here to teach me more about this clue!

'anti-capitalist' is the definition.
I can't tell whether this definition defines the answer.

'harm unknown' is the wordplay.
'harm' becomes 'mar' (I've seen this before).
'unknown' becomes 'x' (algebra).
'mar'+'x'='MARX'

Can you help me to learn more?

(Other definitions for marx that I've seen before include "Red" , "Communist hero" , "Groucho's surname" , "Groucho --, comic" , "The Communist Manifesto author" .)

I've seen this clue in The Sun.

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