Anxiety about horse for slaughter (7)
I believe the answer is:
carnage
'for slaughter' is the definition.
(synonyms)
'anxiety about horse' is the wordplay.
'anxiety' becomes 'care' (care is a kind of anxiety).
'about' means one lot of letters goes inside another.
'horse' becomes 'nag' (term for an old horse).
'care' going around 'nag' is 'CARNAGE'.
(Other definitions for carnage that I've seen before include "Mayhem" , "Bloodshed" , "Bloody slaughter as in battle" , "Widespread killing" , "destruction" .)