Buccaneer or plagiarist (6)
I believe the answer is:
pirate
I believe this is a double definition.
'buccaneer' is the first definition.
(a buccaneer is a pirate)
'plagiarist' is the second definition.
(I know that plagiarist can be written as pirate)
(Other definitions for pirate that I've seen before include "Cornishman maybe" , "Robber on the high scas" , "Hook for one" , "Rover" , "brigand" .)