Head of state (5)
I believe the answer is:
ruler
'head of state' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ruler that I've seen before include "Stationery item" , "Sovereign - support for a firm line!" , "Monarch or yardstick" , "Sovereign, for example" , "One who commands - often seen in the classroom" .)