Desert fault (6)
I believe the answer is:
defect
I believe this clue is a double definition.
'desert' is the first definition.
('defect' can be a synonym of 'desert')
'fault' is the second definition.
(I've seen this before)
(Other definitions for defect that I've seen before include "change allegiance" , "Failing - desert" , "Lack" , "Desert and join the other side" , "bug" .)