Change sides? That’s a fault (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)
'that's a fault' is the second definition.
(synonyms)
(Other definitions for defect that I've seen before include "Blemish, fault" , "go over to the other side" , "change allegiance" , "Leave the country" , "Imperfection - go over to the enemy" .)