Change sides and it’s your fault (6)

I believe the answer is:

defect

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

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

(Other definitions for defect that I've seen before include "Fault - default" , "To change sides dishonourably" , "'Flaw, imperfection (6)'" , "Flaw - change sides" , "Transfer allegiance to opposition" .)

I've seen this clue in the Evening Standard.

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