Rat Scabies finally backing metal singer (8)

I believe the answer is:

snitcher

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'rat' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'scabies finally backing metal singer' is the wordplay.
'finally' indicates one should take the final letters.
'backing metal' becomes 'nit' (I can't justify this - if you can you should believe this answer much more).
'singer' becomes 'cher' (American singer).
The last letter of 'scabies' is 's'.
's'+'nit'+'cher'='SNITCHER'

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I've seen this clue in The Guardian.

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