A group of Marines that helps you stay afloat? (7)
I believe the answer is:
armband
'that helps you stay afloat?' is the definition.
(armbands help the wearer float)
'a group of marines' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'marines' could be 'rm' (abbreviation for 'Royal Marines') and 'rm' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for armband that I've seen before include "Inflatable aid for those learning to swim" , "Worn by football captain or mourner" , "It may identify" , "addition to jacket?" , "part of the uniform" .)