They may be used to measure kings and queens (6)
I believe the answer is:
'kings and queens' is the definition.
(I've seen this in another clue)
'they may be used to measure' is the wordplay.
I cannot really understand how this works, but
'measure' could be 'rule' (rule is a kind of measure) and 'rule' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for rulers that I've seen before include "Strips with straight edge to measure", "Sovereigns and marked sticks", "Sticks marked off in units", "Those in power", "Kings, queens, presidents etc".)