The most common and most likely answer to this crossword clue is the 6 letter word BANIAN
B
A
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More clues leading to the result BANIAN
Buddy explains!

The straight part
We know that "jacket" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jacket = banian.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 BANIAN mean?
BANIAN noun- An Indian trader, merchant, cashier, or money changer.
- A tropical Indian fig tree, Ficus benghalensis, that has many aerial roots.
- A type of loose gown worn in India.
- A vest; an undershirt; a singlet.
- (Naval slang) A camping excursion on shore, to give a ship's crew a break from shipboard routine.