Alloy used for farthings, one imagines (5)

I believe the answer is:

brass

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'alloy' is the definition.
(brass is a kind of alloy)

I can't explain the rest of the clue.

(Other definitions for brass that I've seen before include "Alloy; effrontery" , "Zinc-copper alloy - cheek" , "Instruments, alloy" , "Rhino" , "Boldness" .)

I've seen this clue in the Evening Standard.

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