An incredibly great flier (3)

I believe the answer is:

roc

I'm a little stuck... Click here to teach me more about this clue!

'flier' is the definition.
(I have seen 'Legendary flier' mean 'roc' so perhaps 'flier' could also mean 'roc')

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for roc that I've seen before include "Arabian Nights bird" , "Legendary flier" , "Bird imagined" , "Old bird" , "Gigantic bird of eastern legend - cor!" .)

I've seen this clue in the Evening Standard.

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