How many letters does the top answer for “involving the fourth and no higher power of a quantity or degree” have?

The most common answer is biquadratic (11 letters).

Crossword clue: involving the fourth and no higher power of a quantity or degree

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

biquadratic 11 letters wn-def

a polynomial of the fourth degree