Fancy taking a girl out to swing (4)

I believe the answer is:

vary

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'swing' is the definition.
(both can mean to fluctuate)

I cannot understand how the rest of the clue works.

(Other definitions for vary that I've seen before include "Make a change in" , "Make changes to" , "Fluctuate" , "Differ in size" , "Change, diverge, deviate" .)

I've seen this clue in The Hindu.

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