That crossword clue shows up all the time. They usually want the title character's name, MOBY DICK, since he's the big white whale. But sometimes they get sneaky with it and the answer is just 'AHAB' or 'ISMAEL' if they're asking for a character.
I remember getting stuck on a clue once that was something like 'Melville novel about a scrivener' and I sat there forever before I remembered 'Bartleby the Scrivener'. So if it's vague, think about famous lines or settings too—'MOBY DICK' is the safe bet nine times out of ten, though.
Honestly, it's probably that. The puzzle makers know most people will only have read the one book of his in school.