By tampering with, is cheating (8)

I believe the answer is:

fiddling

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'by' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?

I don't understand the remainder of the clue.

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(Other definitions for fiddling that I've seen before include "Tinkering - trifling" , "petty" , "Cooking the books" , "wasting time?" , "What Nero was doing while Rome burned" .)

I've seen this clue in the Evening Standard.

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