Warning about a little lost fish (4)
I believe the answer is:
nemo
'a little lost fish' is the definition.
(from the film Finding Nemo)
'warning about' is the wordplay.
'warning' becomes 'omen' (I've seen this before).
'about' says the letters should be written in reverse.
'omen' written backwards gives 'NEMO'.
(Other definitions for nemo that I've seen before include "commander for the Nautilus" , "Captain brought to book" , "Fish sought in film" , "Captain of the Nautilus" , "Jules Verne captain" .)