Spoil sheep that's upset (3)
I believe the answer is:
mar
'spoil' is the definition.
('mar' can be a synonym of 'spoil')
'sheep that's upset' is the wordplay.
'sheep' becomes 'ram' (ram is a kind of sheep).
'that's upset' says the letters should be written backwards.
'ram' written backwards gives 'MAR'.
(Other definitions for mar that I've seen before include "Oh dear!" , "Blight" , "when spring growth starts?" , "Spoil, damage" , "Make imperfect" .)