Change sides by 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)
'mistake' is the second definition.
(I've seen this before)
'by' acts as a link.
(Other definitions for defect that I've seen before include "To change sides dishonourably" , "Flaw - change sides" , "Go over to the enemy - it's a flaw" , "Leave the country" , "Deficiency" .)