Proportion in operation (5)

I believe the answer is:

ratio

'proportion' is the definition.
(I know that proportion can be written as ratio)

'in operation' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RATIO' is hidden in the letters of 'operation'.

(Other definitions for ratio that I've seen before include "Relativity between two quantities" , "Proporation, quantitative relation" , "Percentage" , "Proportion between two numbers" , "Relative proportion" .)

I've seen this clue in The Times.

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