Crossword Genius

Big bird to be unsteady, mostly (3)

Ross

I believe the answer is:

roc

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'big bird' is the definition.
(mythical giant bird)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for roc that I've seen before include "Large bird of legend" , "Sinbad's giant bird" , "Vast legendary bird" , "Fabulous giant bird of Eastern legend" , "Sinbad's monster bird" .)

I've seen this clue in The Times.
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