How many letters does the top answer for “a garment worn under other garments” have?

The most common answer is undergarment (12 letters).

Crossword clue: a garment worn under other garments

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

undergarment 12 letters wn-def

a garment worn under other garments

unmentionable 13 letters wn-def

a garment worn under other garments

a garment worn over other garments 100% similar → overgarment