Big bird of myth (3)
I believe the answer is:
roc
'big bird of myth' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roc that I've seen before include "Fabulous Eastern bird of some ferocity" , "Sinbad's giant bird" , "Vast legendary bird" , "Huge mythical bird of prey" , "Giant mythical bird" .)