Only a small part of England has a farm animal on (4)

I believe the answer is:

oxon

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'only a small part of england' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'a farm animal on' is the wordplay.
'a farm animal' becomes 'ox' (castrated ball).
'ox'+'on'='OXON'

'has' is the link.

Can you help me to learn more?

(Other definitions for oxon that I've seen before include "county in brief" , "indefinitely to get half of 24y" , "Oxfordshire, postally" , "Oxfordshire (abbr)" , "English county shortly" .)

I've seen this clue in the Evening Standard.

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