Bird in frock (3)

I believe the answer is:
roc
'bird' is the definition.
(mythological bird of prey)
'in frock' is the wordplay.
'in' indicates a hidden word.
'ROC' can be found hidden inside 'frock'.
(Other definitions for roc that I've seen before include "Bird imagined" , "Gigantic bird of Arabian legend" , "Sinbad's monster bird" , "Huge mythical bird of prey" , "Fantastic bird" .)