Ones playing the game believe in games kit (10)
I believe the answer is:
sportswear
'kit' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sportswear that I've seen before include "Gear" , "in which triathletes swim" , "Kit for games" , "clothes for play" , "Clothing" .)