How many letters does the top answer for “the trait of remaining calm and seeming not to care; a casual lack of concern” have?

The most common answer is nonchalance (11 letters).

Crossword clue: the trait of remaining calm and seeming not to care; a casual lack of concern

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

nonchalance 11 letters wn-def

the trait of remaining calm and seeming not to care; a casual lack of concern

unconcern 9 letters wn-def

the trait of remaining calm and seeming not to care; a casual lack of concern