The most common and most likely answer to this crossword clue is the 9 letter word GOOSANDER
I've seen this crossword clue in Financial Times
G
O
O
S
A
N
D
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Buddy explains!
The straight part
We know that "Duck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case duck = goosander.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GOOSANDER mean?
GOOSANDER noun- A merganser, Mergus merganser, of the northern hemisphere. They eat fish and are common on lakes and rivers.