Crossword Genius

Buoyancy aid for a team of Marines (7)

Ross

I believe the answer is:

armband

This clue might be a double definition.

'buoyancy aid' is the first definition.
(armbands help the wearer stay buoyant)

'team of marines' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'for a' acts as a link.

(Other definitions for armband that I've seen before include "It signals captain" , "Ring for captain" , "Loop for holding up a shirtsleeve" , "Identification worn on sleeve" , "Inflatable aid for those learning to swim" .)

I've seen this clue in The Guardian.
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