Spoil 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 judge whether this definition defines the answer.

'spoil unknown' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'unknown' becomes 'x' (algebra).
'mar'+'x'='MARX'

Can you help me to learn more?

(Other definitions for marx that I've seen before include "Karl -, (Das Kapital)" , "Communist founder" , "Groucho's surname" , "Founder of communism" , "Groucho, Harpo, Chico, Zeppo, Gummo - or Karl?" .)

I've seen this clue in The Sun.

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