Mushroom where batter needs to stand? (8)

I believe the answer is:

increase

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'mushroom' is the definition.
(I know that mushroom is a more specific form of grow)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for increase that I've seen before include "Wax" , "Multiply" , "Increment" , "Augment or enlarge (8)." , "Become greater in number" .)

I've seen this clue in The Times.

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