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Robbed of plumes, it shows some regret (5)

Ross

I believe the answer is:

egret

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'regret' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for egret that I've seen before include "Water-bird, has plume" , "Mainly white heron" , "Heron-like wader" , "White-feathered wading bird" , "Greet (anag) -- wading bird" .)

I've seen this clue in the Evening Standard.
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