Buoyancy aid for a team of Marines (7)
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" .)