A monstrous bird to show instability, one hears (3)

I believe the answer is:

roc

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

'a monstrous bird' is the definition.
(mythical giant bird)

I can't explain the remainder of the clue.

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

I've seen this clue in The Irish News.

Want a hint initially instead of a full solution? Install my app