... must have made joint attack to depose king (4)
I believe the answer is:
need
'must have' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for need that I've seen before include "destitute state" , "Lack - demand" , "Rely on" , "state requiring relief" , "Require (as essential)" .)