Change sides - a mistake (6)

I believe the answer is:

defect

I believe this clue is a double definition.

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

'a mistake' is the second definition.
(I've seen this before)

(Other definitions for defect that I've seen before include "weakness" , "Desert - fault" , "Join opponents in a cause" , "Leave the country" , "Failing or imperfection" .)

I've seen this clue in The Daily Express.

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