Rebuff introduction to Nemo aboard the Nautilus ? (4)
I believe the answer is:
snub
'rebuff' is the definition.
('snub' can be a synonym of 'rebuff')
'introduction to nemo aboard the nautilus ?' is the wordplay.
'introduction to' suggests taking the first letters (I've seen 'introductions to' mean this).
'aboard' indicates putting letters inside.
'the nautilus ?' becomes 'sub' (nautilus is a kind of sub).
The first letter of 'nemo' is 'n'.
'n' inserted within 'sub' is 'SNUB'.
(Other definitions for snub that I've seen before include "Deliberate insult" , "Ignore, spurn disdainfully" , "Check" , "Act of rudeness" , "Reject a person with cold behaviour" .)