To change sides is something wrong (6)

I believe the answer is:

defect

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'to change sides' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for defect that I've seen before include "Desert - flaw" , "go over to the other side" , "Leave the country" , "weakness" , "Blemish, fault" .)

I've seen this clue in The Globe and Mail.

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