Shortcoming (6)
I believe the answer is:
defect
'shortcoming' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for defect that I've seen before include "'Flaw, imperfection (6)'" , "To change sides dishonourably" , "Fault - default" , "Join opponents in a cause" , "go over to the other side" .)