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Record deduction, taking 50% off (4)

Ross

I believe the answer is:

disc

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'record' is the definition.
(eg a CD, compact disc)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for disc that I've seen before include "Old record" , "Flat round plate" , "counter" , "Sound recording" , "One of 33, 45 or 78?" .)

I've seen this clue in The Independent.
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