How many letters does the top answer for “musical instrument with black and white keys” have?

The most common answer is piano (5 letters).

Crossword clue: musical instrument with black and white keys

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

piano 5 letters orig-v32

a keyboard instrument that is played by depressing keys that cause hammers to strike tuned strings and produce sounds