Abandon cause that shows flaw (6)
I believe the answer is:
defect
This clue is a double definition.
'abandon cause' is the first definition.
'flaw' is the second definition.
('defect' can be a synonym of 'flaw')
'that shows' acts as a link.
(Other definitions for defect that I've seen before include "Desert and join the other side" , "Join opponents in a cause" , "error" , "Desert - flaw" , "Want or imperfection" .)