Mix-up is severe when king goes missing (4)
I believe the answer is:
hash
'mix-up' is the definition.
(thesaurus)
'severe when king goes missing' is the wordplay.
I cannot quite see how this works, but
'when' could be 'as' (eg 'when I walked' means 'as I walked') and 'as' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for hash that I've seen before include "Complete mess" , "Keypad symbol" , "that's smoked a lot" , "reheated meat and vegetables" , "What signifies a number" .)