Hook's nemesis
I believe the answer is:
croc
'hook's nemesis' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for croc that I've seen before include "Long-tailed reptile (abbr)" , "Single-handed mariner's nemesis" , "Snapper" , "Large, bony-plated reptile (abbr)" , "Dangerous reptile" .)