Flyer in frock (3)
I believe the answer is:
roc
'flyer' is the definition.
(I have seen 'Legendary flyer' mean 'roc' so perhaps 'flyer' could also mean 'roc')
'in frock' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ROC' is hidden within 'frock'.
(Other definitions for roc that I've seen before include "Cor! what a fabulous bird" , "Huge mythical bird of prey" , "Big bird" , ".Fabulous bird of the East" , "fabulous bird is seen" .)