Nick, dangerous chap in the main (6)
I believe the answer is:
pirate
I believe this is a double definition.
'nick' is the first definition.
(both can mean to steal)
'dangerous chap in the main' is the second definition.
('the main' is an old name for the sea)
(Other definitions for pirate that I've seen before include "Plagiarise" , "Captain Hook, say" , "operating illegally" , "Appropriate stuff" , "Sea raider" .)