Slaughter horse: worry about that (7)
I believe the answer is:
carnage
'slaughter' is the definition.
(I know that slaughter can be written as carnage)
'horse worry about that' is the wordplay.
'horse' becomes 'nag' (term for an old horse).
'worry' becomes 'care' (eg without a care means without a worry).
'about that' is an insertion indicator.
'nag' going within 'care' is 'CARNAGE'.
(Other definitions for carnage that I've seen before include "Bloodshed" , "Bloodbath" , "Great slaughter" , "Indiscriminate slaughter" , "The killing of many people, slaughter" .)