Sign put up by Verne's captain (4)
I believe the answer is:
nemo
'verne's captain' is the definition.
(I've seen this in another clue)
'sign put up' is the wordplay.
'sign' becomes 'omen' (omen is a kind of sign).
'put up' says the letters should be written backwards.
'omen' reversed gives 'NEMO'.
'by' acts as a link.
(Other definitions for nemo that I've seen before include "- me impune lacessit (motto of Scotland)" , "Literary captain" , "commander for the Nautilus" , "Finding -, animated film" , "Jules Verne captain" .)