Helping Susan finally to leave relationship (5)

I believe the answer is:

ratio

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'relationship' is the definition.
(relationship between two numbers)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ratio that I've seen before include "Proportion between two numbers" , "Relationship between two amounts" , "Mathematical relation of one thing to another" , "Proportional relationship" , "Relationship between two numerical quantities" .)

I've seen this clue in The Independent.

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