What those who wish to eat nuts should do first! (3,8)
I believe the answer is:
get cracking
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for get cracking that I've seen before include "Bestir oneself" , "Prepare to break from shell" , "Act quickly and decisively" , "start flying" , "Begin!" .)