Find humour when swapping sides in bed (4,2)

I believe the answer is:

come by

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'find' is the definition.
(I've seen this before)

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

(Other definitions for come by that I've seen before include "Obtain accidentally - visit informally" , "Find" , "pass" , "Encounter" , "Manage to obtain" .)

I've seen this clue in The Independent.

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