It's legal to crack some walnuts (3)
I believe the answer is:
law
'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 ...'" .)