Crossword Genius

It's legal to crack some walnuts (3)

Ross

I believe the answer is:

law

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'crack some walnuts' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for law that I've seen before include "Rule of conduct" , "Official rule" , "at 16 down [MAN UNITED]" , "Regulation" , "'Possession is nine points of the ...'" .)

I've seen this clue in the Evening Standard.
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