Failing to go over to the other side (6)
I believe the answer is:
defect
I believe this is a double definition.
'failing' is the first definition.
(I've seen this before)
'go over to the other side' is the second definition.
(I've seen this before)
'to' acts as a link.
(Other definitions for defect that I've seen before include "Desert - fault" , "Join opponents in a cause" , "Want or imperfection" , "Lack" , "go over to the other side" .)