A land we abandoned that was occupied by invaders (7)
I believe the answer is:
danelaw
'that was occupied by invaders' is the definition.
(historic part of England governed by Dane laws)
'a land we abandoned' is the wordplay.
'abandoned' indicates an anagram.
'a'+'land'+'we'='alandwe'
'alandwe' with letters rearranged gives 'DANELAW'.
(Other definitions for danelaw that I've seen before include "Area of England settled by Vikings" , "Part of country once" , "old north" , "Viking-occupied England" , "Northern part of England when under foreign rule" .)