Monkey with them, and they go to ground! (4)

I believe the answer is:

nuts

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But I can't explain it at all! Could you help me understand?

(Other definitions for nuts that I've seen before include "Crackers - or what crackers are required for?" , "Barmy (colloq)" , "Stun (anag.)" , "One eats these" , "Fruit - bats" .)

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