Put sign up for submarine captain (4)
I believe the answer is:
nemo
'submarine captain' is the definition.
(character from 20,000 Leagues Under the Sea)
I can't explain the rest of the clue.
(Other definitions for nemo that I've seen before include "- me impune lacessit (motto of Scotland)" , "Captain brought to book" , "Captain of the Nautilus" , "commander for the Nautilus" , "Jules Verne captain" .)