How many letters does the top answer for “the american eider duck.” have?

The most common answer is squamduck (9 letters).

Crossword clue: the american eider duck.

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

squamduck 9 letters web-def

The American eider duck. [Local, U.S.]

down of the eider duck 50% similar → eiderdown

the american scaup duck. 50% similar → troopfowl

the common american eider. 50% similar → wamp