Fierce animal? Nonsense! (4)
I believe the answer is:
bull
This clue is a double definition.
'fierce animal?' is the first definition.
'nonsense' is the second definition.
(I've seen this before)
(Other definitions for bull that I've seen before include "Papal edict - nonsense" , "Sign" , "Slangy lies from an ox?" , "Male cow" , "Dartboard's centre" .)