Crossword Genius

Flyer in painfully cold north (7)

Ross

I believe the answer is:

bittern

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'flyer in painfully cold north' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'cold north' is the wordplay.
'cold' becomes 'bitter' (similar in meaning).
'north' becomes 'N' (synonyms).
'bitter'+'n'='BITTERN'

Can you help me to learn more?

(Other definitions for bittern that I've seen before include "large wader" , "Source of boom" , "found hidden in reed-beds" , "Wading bird" , "'He shall not hear the . . . cry' (Ledwidge)" .)

I've seen this clue in The Sun.
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