Failing, join the opposition (6)

I believe the answer is:

defect

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

I cannot understand how the rest of the clue works.

(Other definitions for defect that I've seen before include "Desert - blemish" , "Transfer allegiance to opposition" , "Failing - desert" , "Join opponents in a cause" , "change allegiance" .)

I've seen this clue in The Telegraph.

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