Specialist in stopping admissions? (9)

I believe the answer is:

locksmith

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'specialist in stopping admissions?' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

This is the entire clue.

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(Other definitions for locksmith that I've seen before include "Security expert" , "Security worker" , "laughing stock" , "opener" , "Yale scholar" .)

I've seen this clue in The Sun.

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