How many letters does the top answer for “the cheerful feeling you have when nothing is troubling you” have?

The most common answer is insouciance (11 letters).

Crossword clue: the cheerful feeling you have when nothing is troubling you

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

insouciance 11 letters wn-def

the cheerful feeling you have when nothing is troubling you

lightheartedness 16 letters wn-def

the cheerful feeling you have when nothing is troubling you

the feeling you have when you have hope 50% similar → hopefulness