How many letters does the top answer for “not fully developed in mature animals” have?

The most common answer is vestigial (9 letters).

Crossword clue: not fully developed in mature animals

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

vestigial 9 letters wn-def

not fully developed in mature animals

not developed, mature, or fully formulated 67% similar → undeveloped

not fully developed or mature; not ripe 67% similar → unripe, unripened

(of animals) fully developed 60% similar → adult, grown, grownup