Hereditary heads of state (8)
I believe the answer is:
monarchs
'hereditary heads of state' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for monarchs that I've seen before include "Hereditary rulers" , "Kings, queens and emperors" , "Kings and royal rulers" , "Kings, queens etc." , "Hereditary heads of state" .)