Large ferocious fish (5)
I believe the answer is:
shark
'large ferocious fish' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for shark that I've seen before include "Predatory 5 dn" , "Predatory fish (or person?)" , "Predatory creature" , "Large ferocious fish" , "Voracious fish" .)