Sinbad's giant bird (3)
I believe the answer is:
roc
'sinbad's giant bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for roc that I've seen before include "Mythical bird; sounds like boulder" , "Enormous legendary bird" , "Legendary flier" , "Vast legendary bird" , "Bird imagined" .)