A wedding party usher's back for diamonds scam (9)

I believe the answer is:

deception

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'scam' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's a link between them I don't understand?

I can't explain the rest of the clue.

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(Other definitions for deception that I've seen before include "Illusion" , "Fraud" , "Duplicity" , "Trick" .)

I've seen this clue in the Sydney Morning Herald.

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