Flighty type of big pale horse unseated one (5,5,5)

I believe the answer is:

great white heron

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'flighty type' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'big pale horse unseated one' is the wordplay.
'big' becomes 'great' (synonyms).
'pale' becomes 'white' (pale in colour).
'horse' becomes 'H' (abbreviation as in horse power).
'unseated one' becomes 'eron' (I can't explain this - if you can you should believe this answer much more).
'great'+'white'+'h'+'eron'='GREAT WHITE HERON'

'of' acts as a link.

Can you help me to learn more?

I've seen this clue in The Times and The Sunday Times.

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