Imperfection, defect (4)
I believe the answer is:
flaw
I believe this is a double definition.
'imperfection' is the first definition.
(flaw is a kind of imperfection)
'defect' is the second definition.
(I know that defect can be written as flaw)
(Other definitions for flaw that I've seen before include "Weakness" , "Error in theory or plan" , "weak spot" , "Imperfection or blemish" , "Mark, blemish" .)