Marlin's son, in a Pixar movie (4)
I believe the answer is:
nemo
'marlin's son in a pixar movie' 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 of the Nautilus" , "Fish sought in film" , "commander for the Nautilus" , "- me impune lacessit (motto of Scotland)" , "Captain brought to book" .)