One of two is half expressing mutual relationship (10)

I believe the answer is:

reciprocal

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'relationship' is the definition.
The answer is an example.

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

(Other definitions for reciprocal that I've seen before include "Cross" , "with give and take" , "Mutual" , "Done in return" .)

I've seen this clue in The Guardian.

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