The most common and most likely answer to this crossword clue is the 7 letter word ARMBAND
I've seen this crossword clue in The Guardian
A
R
M
B
A
N
D
More clues leading to the result ARMBAND
- Evidence of authority to issue weapons to troop
- A Royal Marines orchestra’s form of identification
- Military group drops unknown flotation device
- Frenchman grasping black buoyancy aid
- Ring for captain to supply troop with weapons?
- Might this identify a Royal Marines troop?
- Supply troop with augmentation to uniform
- Insignia worn by almost all military musicians
- Member leading group of players? This may confirm it
- A Royal Marines orchestra gets help for swimmer
Buddy explains!

The straight part
We know that "buoyancy aid" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case buoyancy aid = armband.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 ARMBAND mean?
ARMBAND noun- A band worn around the arm, usually to symbolize protest or mourning.
- A band worn around the arm of the captain of a team.
- An inflatable band worn round the arms to keep afloat in water