The most common and most likely answer to this crossword clue is the 8 letter word MACKINAW
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The straight part
We know that "coat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case coat = mackinaw.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 MACKINAW mean?
MACKINAW noun- A heavy woolen cloth.
- A blanket made of wool, formerly distributed to the Amerindians by the U.S. government.
- A flat-bottomed cargo boat; mackinaw boat.