Desert by mistake (6)
I believe the answer is:
defect
I believe this is a double definition.
'desert' is the first definition.
(synonyms)
'mistake' is the second definition.
(I've seen this before)
'by' acts as a link.
(Other definitions for defect that I've seen before include "weakness" , "mistake" , "Desert - flaw" , "error" , "Desert - blemish" .)