A sign that Verne's captain has returned (4)
I believe the answer is:
omen
'a sign' is the definition.
(I know that omen is a type of sign)
'verne's captain has returned' is the wordplay.
'verne's captain' becomes 'nemo' (I've seen this in another clue).
'has returned' says the letters should be written in reverse.
'nemo' reversed gives 'OMEN'.
'that' is the link.
(Other definitions for omen that I've seen before include "Sign of something to come" , "Augur" , "Forewarning, harbinger" , "Straw in the wind" , "premonition" .)