In a while, get baggy (7)

I believe the answer is:

stretch

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'in' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for stretch that I've seen before include "Exercise - period" , "extended" , "Bit of exercise" , "Make to go further" , "Make longer/wider" .)

I've seen this clue in the Evening Standard.

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