How many letters does the top answer for “a chimney cap; a cowl” have?

The most common answer is cow (3 letters).

Crossword clue: a chimney cap; a cowl

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

cow 3 letters web-def

female of domestic cattle: "`moo-cow' is a child's term"