Club it could be shocking for a king to join (4)
I believe the answer is:
cosh
'club' is the definition.
(cosh is a kind of club)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for cosh that I've seen before include "Sap" , "Truncheon, bludgeon" , "Thick heavy stick as a weapon" , "Club used as a weapon" , "Blunt weapon, often of hard rubber" .)