Robber at sea (6)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
pirate
'robber at sea' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pirate that I've seen before include "Hook for one" , "Captain Hook, say" , "Criminal on the high seas" , "Teach, perhaps" , "Illegal" .)