Failing, change sides (6)
I believe the answer is:
defect
I believe this clue is a double definition.
'failing' is the first definition.
(thesaurus)
'change sides' is the second definition.
(thesaurus)
(Other definitions for defect that I've seen before include "Deficiency" , "Desert - fault" , "weakness" , "Desert - flaw" , "bug" .)