Viking invader overrunning southeast (8)
I believe the answer is:
norseman
'viking' is the definition.
(I've seen this before)
'invader overrunning southeast' is the wordplay.
'invader' becomes 'norman' (norman conquest).
'overrunning' indicates putting letters inside.
'southeast' becomes 'se' ('SE' can be a synonym of 'southeast').
'norman' enclosing 'se' is 'NORSEMAN'.
(Other definitions for norseman that I've seen before include "one attacking north-east?" , "invader" , "Scandinavian" , "A viking, perhaps" .)