The fictional captain takes it to be a habit (4)
I believe the answer is:
ahab
This could be a double definition.
'the fictional captain' is the first definition.
(I've seen this in another clue)
'it to be a habit' is the second definition.
The definition suggests a singular noun which matches the answer.
'takes' acts as a link.
Can you help me to learn more?
(Other definitions for ahab that I've seen before include "Jezebel's husband or captain in 'Moby Dick'" , "Name of captain seeking Moby Dick" , "Husband of Jezebel" , "Captain in search of Moby Dick" , "Captain -, pursuer of Moby Dick" .)