How many letters does the top answer for “the absence of a key; alternative to the diatonic system” have?

The most common answer is atonalism (9 letters).

Crossword clue: the absence of a key; alternative to the diatonic system

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

atonalism 9 letters wn-def

the absence of a key; alternative to the diatonic system

atonality 9 letters wn-def

the absence of a key; alternative to the diatonic system