The measure of a king (5)
I believe the answer is:
ruler
This clue is a double definition.
'the measure' is the first definition.
(ruler is a kind of measure)
'a king' is the second definition.
(I've seen this before)
'of' is the link.
(Other definitions for ruler that I've seen before include "Person commanding" , "One measures" , "Sovereign, for example" , "Head of state" , "Graduated straight-edge" .)