Change sides? That's weakness (6)
I believe the answer is:
defect
I believe this is a double definition.
'change sides?' is the first definition.
(I've seen this before)
'weakness' is the second definition.
(a defect is a flaw or weakness)
'that's' acts as a link.
(Other definitions for defect that I've seen before include "'Flaw, imperfection (6)'" , "Blemish, fault" , "Shortcoming" , "Lack" , "Failing or imperfection" .)