Decorative item beginning to sparkle for a horse, not tail (6)
I believe the answer is:
sequin
'tail' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sequin that I've seen before include "Sparkling feature of ballroom dancing?" , "bright little thing" , "Small shiny disc for decoration" , "Small shiny disc on a garment" , "Small circular spangle for ornamenting clothing" .)