How many letters does the top answer for “to swim under water; -- said of air-breathing creatures.” have?

The most common answer is mill (4 letters).

Crossword clue: to swim under water; -- said of air-breathing creatures.

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

mill 4 letters web-def

a plant consisting of one or more buildings with facilities for manufacturing