Silly-billy I spot mainlining premier bit of ice (5)

I believe the answer is:

idiot

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'silly' is the definition.
(thesaurus)

I don't understand the remainder of the clue.

(Other definitions for idiot that I've seen before include "Imbecile" , "Silly person or dimwit" , "Village character" , "Mug" , "Dipstick" .)

I've seen this clue in The Independent.

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