How many letters does the top answer for “a piece of writing in code or cipher” have?

The most common answer is cryptogram (10 letters).

Crossword clue: a piece of writing in code or cipher

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

cryptogram 10 letters wn-def

a piece of writing in code or cipher

act of writing in code or cipher 60% similar → coding, cryptography, steganography