Commander of the Nautilus
I believe the answer is:
'commander of the nautilus' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nemo that I've seen before include "Captain brought to book" , "Finding -, animated film" , "Jules Verne captain" , "- me impune lacessit (motto of Scotland)" , "Jules Verne's submarine captain" .)