Change sides by mistake (6)
I believe the answer is:
defect
I believe this 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' is the 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" .)