Drive forcefully into separate defensive wall (7)
I believe the answer is:
rampart
'defensive wall' is the definition.
(I know that rampart is a type of defensive wall)
'drive forcefully into separate' is the wordplay.
'drive forcefully into' becomes 'ram' (as in ramming into something).
'separate' becomes 'part' (both can mean to split or break apart).
'ram'+'part'='RAMPART'
(Other definitions for rampart that I've seen before include "means of defence" , "Fortified wall" , "Defensive structure" , "Bulwark, wall" , "defencework" .)