Idiot gets B twice using stringed instrument (10)

I believe the answer is:

doublebass

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'idiot gets b' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'twice using stringed instrument' is the wordplay.
'twice' becomes 'double' (synonyms).
'using' is a charade indicator (letters next to each other).
'stringed instrument' becomes 'bass' (I can't justify this - if you can you should believe this answer much more).
'double'+'bass'='DOUBLEBASS'

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I've seen this clue in The Irish News.

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