Sheep rolling over is hurt (3)
I believe the answer is:
mar
'is hurt' is the definition.
(I've seen this in other clues)
'sheep rolling over' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'rolling over' says the letters should be written in reverse.
'ram' back-to-front is 'MAR'.
(Other definitions for mar that I've seen before include "Disfigure" , "Cause harm" , "'Damage, impair (3)'" , "Blemish" , "Oh dear!" .)