Yardstick for a crowned head (5)
I believe the answer is:
ruler
I believe this is a double definition.
'yardstick' is the first definition.
(I've seen this in another clue)
'crowned head' is the second definition.
(I've seen this in another clue)
'for a' is the link.
(Other definitions for ruler that I've seen before include "Maharaja, e.g." , "Straight edge; monarch" , "It can be used to strike" , "king perhaps" , "One exercises dominion" .)