A mistake to change sides? (6)
I believe the answer is:
defect
This clue is a double definition.
'a mistake' is the first definition.
(I've seen this before)
'change sides?' is the second definition.
(thesaurus)
'to' is the link.
(Other definitions for defect that I've seen before include "Want or imperfection" , "go over to the other side" , "Desert - blemish" , "Fault, flaw" , "bug" .)