For being a bit fierce, shame on you! (3)

I believe the answer is:

fie

I'm a little stuck... Click here to teach me more about this clue!

'shame on you' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

(Other definitions for fie that I've seen before include "Yuck!" , "Interjection of disgust" , "Exclamation of mock dismay" , "Cry of dismay" , "Shame!" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app