Proportion shown as harangue avoids extremes (5)

I believe the answer is:

ratio

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ratio that I've seen before include "Relative magnitude" , "Comparative size" , "Quotient, proportion" , "Quotient of two numbers" , "9 to 5, say?" .)

I've seen this clue in The Independent.

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