How many letters does the top answer for “old form of have” have?

The most common answer is hast (4 letters).

Crossword clue: old form of have

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

hast 4 letters orig-v32

, 2d pers. sing. pres. of. Fave, contr. of havest. [Archaic]

an old form of be and been. 50% similar → bin

an old form of elf. 50% similar → elve

old form of does 50% similar → doth