How many letters does the top answer for “not wearing a jacket” have?

The most common answer is shirtsleeves (12 letters).

Crossword clue: not wearing a jacket

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

shirtsleeves 12 letters wn-def

not wearing a jacket

not wearing a hat 50% similar → hatless

not wearing a wig 50% similar → wigless

wearing, or furnished with, a jacket. 50% similar → jacketed