Bird in frock (3)
I believe the answer is:
roc
'bird' is the definition.
(mythological bird of prey)
'in frock' is the wordplay.
'in' says the answer is hidden in the clue.
'ROC' is hidden within '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" .)