Sheep on some defensive wall (7)
I believe the answer is:
rampart
'defensive wall' is the definition.
(I know that rampart is a type of defensive wall)
'sheep on some' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'on' is a charade indicator (letters next to each other).
'some' becomes 'part'.
'ram'+'part'='RAMPART'
(Other definitions for rampart that I've seen before include "barricade" , "Embankment built for defensive purposes" , "Battlement, fortification" , "means of defence" , "Fortified wall" .)