Old cavalry officers lacking nothing once in ornamental headwear (7)

I believe the answer is:

cornets

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'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.

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(Other definitions for cornets that I've seen before include "Brass instruments" , "Ice-cream cones" .)

I've seen this clue in The Independent.

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