Worry about horse causing bloodshed (7)
I believe the answer is:
carnage
'bloodshed' is the definition.
(I've seen this in another clue)
'worry about horse' is the wordplay.
'worry' becomes 'care' (eg without a care means without a worry).
'about' means one lot of letters goes inside another.
'horse' becomes 'nag' (term for an old horse).
'care' placed around 'nag' is 'CARNAGE'.
'causing' is the link.
(Other definitions for carnage that I've seen before include "Widespread killing" , "Bloodbath" , "Mass slaughter" , "Killing of large numbers" , "Wholesale killing" .)