Batter some of the defence (7)
I believe the answer is:
rampart
'the defence' is the definition.
(rampart is a kind of defence)
'batter some' is the wordplay.
'batter' becomes 'ram' (I've seen this before).
'some' becomes 'part' (only part of something).
'ram'+'part'='RAMPART'
'of' is the link.
(Other definitions for rampart that I've seen before include "Battlement, fortification" , "Defensive wall of a castle" , "barricade" , "Pain-relieving drug" , "Castle embankment" .)