Poles surround ruined manor, invaders (7)
I believe the answer is:
normans
'invaders' is the definition.
(Norman Conquest)
'poles surround ruined manor' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'surround' means one lot of letters goes inside another.
'ruined' indicates an anagram.
'manor' with letters rearranged gives 'orman'.
'ns' going around 'orman' is 'NORMANS'.
(Other definitions for normans that I've seen before include "They won the battle of Hastings" , "belonging to Greg the golfer?" , "Ten sixty six invaders" , "French folk" , "William the Conqueror's lot?" .)