Flippantly changing sides at the start according to the rules (7)

I believe the answer is:

rightly

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'flippantly' is the definition.
Both the answer and definition are adverbs. Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

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(Other definitions for rightly that I've seen before include "Justly, correctly" , "With good reason" , "Honestly or justifiedly" , "Justly, honestly" .)

I've seen this clue in The Independent.

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