Butchery had to bother about old horse (7)

I believe the answer is:

carnage

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'butchery' is the definition.
(I know that butchery can be written as carnage)

'to bother about old horse' is the wordplay.
'to bother' becomes 'nag' (I've seen this before).
'about' is an insertion indicator.
'old horse' becomes 'care' (I can't explain this - if you can you should believe this answer much more).
'nag' inserted into 'care' is 'CARNAGE'.

'had' is the link.

Can you help me to learn more?

(Other definitions for carnage that I've seen before include "The killing of many people, slaughter" , "Indiscriminate slaughter" , "Blood bath" , "Bloodbath" , "Savage killing of people" .)

I've seen this clue in the Sydney Morning Herald.

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