Robbed of plumes, it shows some regret (5)
I believe the answer is:
egret
'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" .)