Large bird is somewhat ferocious (3)
I believe the answer is:
roc
'large bird' is the definition.
(mythical giant bird)
'somewhat ferocious' is the wordplay.
'somewhat' indicates the answer is hidden within the clue.
'ROC' is hidden within 'ferocious'.
'is' acts as a link.
(Other definitions for roc that I've seen before include "Gigantic bird of eastern legend - cor!" , "Bird imagined" , "Sinbad's huge bird" , "Fabulous giant bird of Eastern legend" , "Legendary air carrier" .)