To others on stage, it's unheard of! (5)

I believe the answer is:

aside

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'to' is the definition.
Both the definition and answer are adverbs.
Perhaps you can see an association between them that I don't see?

I don't understand the remainder of the clue.

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(Other definitions for aside that I've seen before include "Remark not for hearing" , "Line spoken to theatre audience" , "Words spoken apart in an undertone" , "Stage whisper" , "Apart; incidental remark" .)

I've seen this clue in the Evening Standard.

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