How many letters does the top answer for “causing or able to cause motion” have?

The most common answer is motor (5 letters).

Crossword clue: causing or able to cause motion

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

motor 5 letters wn-def

machine that converts other forms of energy into mechanical energy and so imparts motion

causing or able to cause nausea 60% similar → nauseating, sickening