Number with hatred for decimalisation? (5)

I believe the answer is:

opium

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'number' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for opium that I've seen before include "wars with China" , "Narcotic derived from poppy" , "where horse originally came from?" , "Narcotic derived from poppies" , "Narcotic drug" .)

I've seen this clue in The Independent and The Sun.

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