Someone acting as a stopgap (10)

I believe the answer is:

understudy

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'someone' is the definition.
(understudy is a kind of someone)

I don't understand how the rest of the clue works.

(Other definitions for understudy that I've seen before include "reserve?" , "one covers part" , "One prepared to take over from leading actor" , "emergency role" , "Replacement" .)

I've seen this clue in the Evening Standard.

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