Worry about horse butchery (7)
I believe the answer is:
carnage
'butchery' is the definition.
('carnage' can be a synonym of 'butchery')
'worry about horse' is the wordplay.
'worry' becomes 'care' (I've seen this before).
'about' means one lot of letters goes inside another.
'horse' becomes 'nag' (nag is a kind of horse).
'care' placed around 'nag' is 'CARNAGE'.
(Other definitions for carnage that I've seen before include "Bloodshed" , "Killing of large numbers" , "Widespread killing" , "Multiple killing" , "Bloody slaughter as in battle" .)