Monkey with them, and they go to ground! (4)
I believe the answer is:
nuts
'ground' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?
'monkey with them and they go' is the wordplay.
I cannot quite understand how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is present in the answer.
The remaining letters 'uts' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(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" .)