Crack some walnuts, as a rule (3)
I believe the answer is:
law
'a rule' is the definition.
(law is a kind of rule)
'crack some walnuts' is the wordplay.
'crack' becomes 'la' (I am not sure about this - if you are sure you should believe this answer much more).
'some' suggests taking the first letters.
The initial letter of 'walnuts' is 'w'.
'la'+'w'='LAW'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for law that I've seen before include "Rule, statute" , "Commandment" , "Official rule" , "Set of rules laid down by authority" , "Edict" .)