Captain of the Pequod (4)
I believe the answer is:
'captain of the pequod' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ahab that I've seen before include "Jezebel's husband, wicked king in the 'Old testament'" , "Captain -, pursuer of Moby Dick" , "Israelite king" , "one who hunted whales" , "sea captain" .)