Obsessed captain (4)
I believe the answer is:
'obsessed captain' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ahab that I've seen before include "Captain of the whaler Pequod" , "Melville's Captain" , "Name of captain seeking Moby Dick" , "Captain hunting Moby Dick" , "Ancient king of Israel" .)