How many letters does the top answer for “one who writes a treatise.” have?

The most common answer is treatiser (9 letters).

Crossword clue: one who writes a treatise.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

treatiser 9 letters web-def

a formal exposition

one who writes a monograph. 50% similar → monographist

one who writes novels 50% similar → novelist

one who writes on heresies. 50% similar → heresiographer