Understanding one's having an affair with (6)

I believe the answer is:

seeing

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'understanding' is the definition.
('see' can be a synonym of 'understand')

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for seeing that I've seen before include ". . . . . . is believing, they say" , "since" , "Perceiving by eye" , "Meeting" , "Observing - understanding" .)

I've seen this clue in the Evening Standard.

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