Six for Roman ruler or raider (6)
I believe the answer is:
viking
'raider' is the definition.
(I have seen 'Scandinavian raider ' mean 'viking' so perhaps 'raider' could also mean 'viking')
'six for roman ruler' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'for' is a charade indicator (letters next to each other).
'roman ruler' becomes 'king' (king is a kind of ruler. I am not sure about the 'roman' bit.).
'vi'+'king'='VIKING'
'or' is the link.
Can you help me to learn more?
(Other definitions for viking that I've seen before include "25 28 [MARS LANDER]" , "Ancient Scandinavian adventurer" , "Scandinavian warrior" , "Scandinavian raider" , "One of the early invaders of Ireland" .)