Robbery at sea (6)
I believe the answer is:
piracy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">robbery at sea</span>' is the definition.<br/ ><span class="explanation-format__explanation">(what pirates do)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for piracy that I've seen before include "Robbing of ships at sea" , "Robbery at sea or of copyright" , "theft" , "Act of plagiarism" , "Maritime or commercial robbery" .)