Huge mythical bird (3)
I believe the answer is:
roc
'huge mythical bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for roc that I've seen before include "Huge mythical bird of prey" , "Legendary flier" , "Cor! what a fabulous bird" , "Gigantic bird of Arabian legend" , "Sinbad's giant bird" .)