Six rulers are Norsemen (7)
I believe the answer is:
vikings
'are norsemen' is the definition.
(historical Norse people)
'six rulers' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'rulers' becomes 'kings' (king is a kind of ruler).
'vi'+'kings'='VIKINGS'
(Other definitions for vikings that I've seen before include "Scandinavian adventurers who attacked Ireland" , "Scandinavian marauders" , "Invaders" , "Norsemen" .)