The most common and most likely answer to this crossword clue is the 14 letter word STALKING-HORSE
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Buddy explains!
The straight part
We know that "Decoy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decoy = stalking-horse.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 STALKING-HORSE mean?
STALKING-HORSE noun- A horse used as cover by a hunter stalking game.
- A candidate put forward to serve a hidden, ulterior purpose in a political campaign, such as testing the field for another potential candidate by gauging voter sentiment or covertly helping another candidate by attracting voters away from a third candidate.
- (by extension) A person, thing, or expedient used in a deceptive manner, to achieve some hidden purpose.