Old cavalry officers lacking nothing once in ornamental headwear (7)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
cornets
'old cavalry officers' is the definition.
Although both the answer and definition are plural nouns, I don't understand how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cornets that I've seen before include "Brass instruments" , "Ice-cream cones" .)