It's instrumental in revealing boastfulness (7)

I believe the answer is:

trumpet

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'boastfulness' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

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

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(Other definitions for trumpet that I've seen before include "What elephants do" , "Horn" , "Brass wind instrument" , "Loudly sound - instrument" , "Pert tum (anag.)" .)

I've seen this clue in The Globe and Mail.

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