Norseman versus island ruler (6)
I believe the answer is:
viking
'norseman' is the definition.
(I've seen this before)
'versus island ruler' is the wordplay.
'versus' becomes 'v'.
'island' becomes 'i' (geographical abbreviation).
'ruler' becomes 'king' (king is a kind of ruler).
'v'+'i'+'king'='VIKING'
(Other definitions for viking that I've seen before include "25 28 [MARS LANDER]" , "Scandinavian warrior" , "Ancient Scandinavian adventurer" , "Norseman" , "Scandinavian raider" .)