Mako or great white (5)
I believe the answer is:
shark
'mako or great white' is the definition.
(I know that mako is a type of mackerel shark)
This is the entire clue.
(Other definitions for shark that I've seen before include "danger at sea" , "Predatory 5 dn" , "Predatory fish (or person?)" , "Carnivorous fish" , "(Fishy) extortionist" .)