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Mum's a winner at chess! (5)

Ross

I believe the answer is:

mater

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'mum's' is the definition.
(old-fashioned term for mother)

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

Can you help me to learn more?

(Other definitions for mater that I've seen before include "Mum" , "having children" , "parent" , "See 12" , "Mother (sl.)" .)

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