Most hornets, if disturbed, will go flying! (5)

I believe the answer is:

heron

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

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for heron that I've seen before include "flier" , "Rhone (anag) - bird" , "eg Egret" , "Big bird" , "Bird of the Rhone, strangely" .)

I've seen this clue in the Evening Standard.

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