Robbery on the high seas (6)
I believe the answer is:
piracy
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">robbery on the high seas</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for piracy that I've seen before include "Robbing of ships at sea" , "Illegal copying" , "Act of plagiarism" , "Robbery on the high seas" , "Crime" .)