Crossword Genius

Three keys in a carrier (3)

Ross

I believe the answer is:

bag

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'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)" .)

I've seen this clue in The Mirror.
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