Fault and default (6)
I believe the answer is:
defect
'default' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for defect that I've seen before include "Deficiency" , "Leave the country" , "weakness" , "Want or imperfection" , "Flaw; turn traitor" .)