The most common and most likely answer to this crossword clue is the 6 letter word TURKEY
I've seen this crossword clue in The Times
T
U
R
K
E
Y
More clues leading to the result TURKEY
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The straight part
We know that "dead duck" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dead duck = turkey.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 TURKEY mean?
TURKEY noun- The guinea fowl (family Numididae).
- A bird in the genus Meleagris with a fan-shaped tail and wattled neck, especially the wild turkey (Meleagris gallopavo, now domesticated).
- The flesh or meat of this bird eaten as food.
- With a distinguishing word: a bird resembling the Meleagris gallopavo (for example, the bush turkey or brush turkey (Alectura lathami), and the water turkey (Anhinga anhinga)).
- An act of throwing three strikes in a row.
- A patient feigning symptoms; a person faking illness or injury; a malingerer.
- A pack carried by a lumberman; a bindle; also, a large travel bag, a suitcase.
- A failure.
- (usually mildly) A foolish or inept person.