How many letters does the top answer for “another word for key” have?

The most common answer is cardinal (8 letters).

Crossword clue: another word for key

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

cardinal 8 letters gen-syn

(Roman Catholic Church) one of a group of more than 100 prominent bishops in the Sacred College who advise the Pope and

fundamental 11 letters gen-syn

the lowest tone of a harmonic series

another word for about 50% similar → approximately, roughly, some

another word for terms 50% similar → footing

another word for under 50% similar → nether

another word for word 50% similar → phrase

to pass from one key into another. 50% similar → modulate