Go over to the Russians? That’s a mistake (6)
I believe the answer is:
defect
This might be a double definition.
'go over' is the first definition.
(to defect is to go over to the other side)
'that's a mistake' is the second definition.
(I've seen this before)
'to the russians?' acts as a link.
This may not be correct. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for defect that I've seen before include "Flaw; turn traitor" , "Flaw - change sides" , "Failing or imperfection" , "Flaw, blemish" , "Abandon one's country in favour of an opposing one" .)