One captivated by cow (mingling with sheep) that's the best in field (9)
I believe the answer is:
showpiece
'field' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for showpiece that I've seen before include "fine specimen of its kind" , "especially fine example" , "Item for exhibition" , "Impressive exhibit" , "Item of work presented for display" .)