Crazy old king’s losing his head (4)
I believe the answer is:
nuts
'crazy' is the definition.
(thesaurus)
'old king's losing his head' is the wordplay.
'old king's' becomes 'cnuts' (King Cnut).
'losing his head' means to remove the first letter.
'cnuts' with its initial letter taken away is 'NUTS'.
(Other definitions for nuts that I've seen before include "Eccentric; coal lumps" , "Slangily crazy like hard fruits" , "Metal fasteners" , "Fruit - bananas!" , "deprived of one's wits" .)