Understanding what the nod of the head means (9)

I believe the answer is:

agreement

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'understanding' is the definition.
(I know that understanding can be written as agreement)

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for agreement that I've seen before include "Harmony or treaty" , "Harmony, concurrence" , "Harmony of opinion" , "match" , "concert" .)

I've seen this clue in the Evening Standard.

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