Win game of chess with friend (4)

I believe the answer is:

mate

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

'friend' is the definition.
(I know that mate is a type of friend)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for mate that I've seen before include "Final move in chess" , "Chess ending; pal" , "Friend, buddy" , "assistant" , "Companion, wife" .)

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