Sheep has damage to back (3)
I believe the answer is:
ram
'sheep' is the definition.
(I've seen this before)
'damage to back' is the wordplay.
'damage' becomes 'mar' (marring is a kind of damaging).
'to back' says the letters should be written in reverse.
'mar' back-to-front is 'RAM'.
'has' acts as a link.
(Other definitions for ram that I've seen before include "force" , "Computers need it to operate" , "Pound" , "Strike violently" , "Temporary computer memory, initially" .)