Crack a salt nut for a frustrated king (8)
I believe the answer is:
tantalus
'frustrated king' is the definition.
'tantalus' can be an answer for 'king' (I have seen 'ancient king' mean 'tantalus' so perhaps 'king' could also mean 'tantalus'). I am not certain of the 'frustrated' bit.
'crack a salt nut' is the wordplay.
'crack' is an anagram indicator.
'a'+'salt'+'nut'='asaltnut'
'asaltnut' anagrammed gives 'TANTALUS'.
'for a' is the link.
Can you help me to learn more?
(Other definitions for tantalus that I've seen before include "Lydian king" , "Lockable case for decanters" , "Frustrated Greek" , "Son of Zeus and case for decanters" , "Secure liquor decanter" .)