Three keys in a carrier (3)
I believe the answer is:
bag
'carrier' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for bag that I've seen before include "Gladstone, for example" , "Reserve (colloq.)" , "Flexible container for shopping" , "Container for carrying thing" , "Item of luggage; catch and kill (an animal)" .)