Great flier, simply fabulous (3)

I believe the answer is:

roc

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'great flier' is the definition.
'roc' can be an answer for 'flier' (I have seen 'Legendary flier' mean 'roc' so perhaps 'flier' could also mean 'roc'). I'm not sure about the 'great' bit.

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

Can you help me to learn more?

(Other definitions for roc that I've seen before include "Legendary thing" , "Vast legendary bird" , "Large bird of legend" , "Mythical creature from Arabia" , "Sinbad's huge bird" .)

I've seen this clue in the Evening Standard.

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