Swimming aid (7)
'swimming aid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for armband that I've seen before include "Cloth worn round sleeve" , "Inflatable aid for those learning to swim" , "What might hold up sleeve" , "Worn by football captain or mourner" , "support for member" .)
(Other definitions for flipper that I've seen before include "Limb used for swimming" , "Seal/turtle limb" , "Before the game, the referee perhaps makes" , "Dolphin's limb" , "Shoe for swimming" .)