Pixar's lost clownfish (4)
I believe the answer is:
nemo
'pixar's lost clownfish' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nemo that I've seen before include "Captain brought to book" , "commander for the Nautilus" , "Captain of the Nautilus" , "Jules Verne's submarine captain" , "Literary captain" .)