How many letters does the top answer for “a genus of plants. see four-o'clock.” have?

The most common answer is mirabilis (9 letters).

Crossword clue: a genus of plants. see four-o'clock.

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

mirabilis 9 letters web-def

four o'clocks

a genus of plants. see rhubarb. 50% similar → rheum

a genus of plants. see yam. 50% similar → dioscorea

a genus of plants; the mandrake. see mandrake, 1. 50% similar → mandragora