How many letters does the top answer for “an old form of be and been.” have?

The most common answer is bin (3 letters).

Crossword clue: an old form of be and been.

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

bin 3 letters web-def

a container; usually has a lid

an old form of elf. 50% similar → elve

old form of does 50% similar → doth

old form of have 50% similar → hast