Pirate victory over independent ruler (6)
I believe the answer is:
viking
'pirate' is the definition.
(I've seen this before)
'victory over independent ruler' is the wordplay.
'victory' becomes 'v'.
'over' means one lot of letters go next to another.
'independent' becomes 'i'.
'ruler' becomes 'king' (king is a kind of ruler**).
'v'+'i'+'king'='VIKING'
(Other definitions for viking that I've seen before include "Scandinavian raider" , "Seafaring Scandinavian" , "One of the early invaders of Ireland" , "Norseman" , "Norse trader, pirate" .)