Old red blemishes on the tongue? (4)

I believe the answer is:

marx

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'old red' is the definition.
'marx' can be an answer for 'red' (I've seen this before). I'm not sure about the 'old' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for marx that I've seen before include "Second part of 20" , "Harpo's surname" , "Communist hero" , "Das Kapital author" , "Founder of communism" .)

I've seen this clue in The Independent.

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