How many letters does the top answer for “the longest river on earth” have?

The most common answer is nile (4 letters).

Crossword clue: the longest river on earth

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

nile 4 letters wd-fact

the world's longest river (4150 miles); flows northward through eastern Africa into the Mediterranean; the Nile River va

the longest river in africa 50% similar → nile