Mangrove swamp snapper (4)
I believe the answer is:
croc
'mangrove swamp snapper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for croc that I've seen before include "Predatory reptile (informal)" , "Small reptile" , "Large predatory reptile (colloq.)" , "Fierce reptile" , "creature heard to be a vicious one" .)