Sheep given role in 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 given role' is the wordplay.
'sheep' becomes 'ram' (male sheep).
'given' means one lot of letters go next to another.
'role' becomes 'part' (eg a part or role in a film).
'ram'+'part'='RAMPART'
'in' is the link.
(Other definitions for rampart that I've seen before include "Castle embankment" , "Bulwark, wall" , "defencework" , "means of defence" , "Battlement, fortification" .)