Embankment built for defensive purposes (7)
I believe the answer is:
rampart
'embankment built for defensive purposes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rampart that I've seen before include "Bulwark, wall" , "defencework" , "Embankment built for defensive purposes" , "Broad defensive wall" , "Defensive wall or mound" .)