How many letters does the top answer for “defending or preserving from disease; preventive. coxe.” have?

The most common answer is prophylactic (12 letters).

Crossword clue: defending or preserving from disease; preventive. coxe.

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

prophylactic 12 letters web-def

contraceptive device consisting of a sheath of thin rubber or latex that is worn over the penis during intercourse

prophylactical 14 letters web-def

Defending or preserving from disease; preventive. Coxe.