Gigantic bird dislodging kittiwakes primarily from crag (3)

I believe the answer is:

roc

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'gigantic bird' is the definition.
(I've seen this before)

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

(Other definitions for roc that I've seen before include "Mythical creature from Arabia" , "Legendary thing" , "Vast legendary bird" , "Enormous, mythical bird of prey" , "Giant mythical bird" .)

I've seen this clue in The Times.

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