Verne's captain back for sign (4)
I believe the answer is:
'for sign' is the definition.
(omen is a kind of sign)
'verne's captain back' is the wordplay.
'verne's captain' becomes 'nemo' (I've seen this in another clue).
'back' says the letters should be written backwards.
'nemo' written backwards gives 'OMEN'.
(Other definitions for omen that I've seen before include "Augur" , "Foreboding" , "Straw in the wind" , "Prognostication" , "Portent, prophetic indicator" .)