How many letters does the top answer for “a falsehood. see lie.” have?

The most common answer is lye (3 letters).

Crossword clue: a falsehood. see lie.

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

lye 3 letters web-def

a strong solution of sodium or potassium hydroxide

of lie, to tell a falsehood. 50% similar → lying