Close to being grand in Wimbledon (4)

I believe the answer is:

slam

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'close' is the definition.
(I know that slam is a type of close)

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

Can you help me to learn more?

(Other definitions for slam that I've seen before include "Hit hard" , "(Almost) all the tricks" , "If it's grand, it wins all 13 tricks in bridge game" , "Shut (a door) noisily" , "Severely criticise; shut (a door) forcefully" .)

I've seen this clue in The Mirror.

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