Moby Dick's domain (5)
I believe the answer is:
ocean
'moby dick's domain' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ocean that I've seen before include "One of the Seven Seas" , "Major sea" , "lot of liquid" , "Sea - canoe (anag)" , "Extensive sea area" .)