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Dubious humming, intro forgotten (4)

Ross

I believe the answer is:

iffy

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'dubious' is the definition.
(I've seen this before)

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

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(Other definitions for iffy that I've seen before include "Dubious, risky (inf.)" , "Informally dubious or uncertain" , "Problematic" , "Unsure" , "Dubious (colloq.)" .)

I've seen this clue in The Guardian.
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