It's only a small part of England (4)
I believe the answer is:
oxon
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for oxon that I've seen before include "county in brief" , "Oxfordshire, postally" , "Oxfordshire (abbr)" , "English county shortly" , "indefinitely to get half of 24y" .)