Mum's a winner at chess! (5)
I believe the answer is:
mater
'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.)" .)