How many letters does the top answer for “working or spreading in a hidden and usually injurious way” have?

The most common answer is insidious (9 letters).

Crossword clue: working or spreading in a hidden and usually injurious way

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

insidious 9 letters wn-def

working or spreading in a hidden and usually injurious way

pernicious 10 letters wn-def

working or spreading in a hidden and usually injurious way

subtle 6 letters wn-def

working or spreading in a hidden and usually injurious way