Go over to the other side? That’s a mistake (6)
I believe the answer is:
defect
This clue is a double definition.
'go over to the other side?' is the first definition.
(I've seen this before)
'a mistake' is the second definition.
(I've seen this before)
'that's' is the link.
(Other definitions for defect that I've seen before include "Join opponents in a cause" , "error" , "Transfer allegiance to opposition" , "Desert - fault" , "To change sides dishonourably" .)