Start to crumble? Not entirely 'zero defects' (5)
I believe the answer is:
erode
'start to crumble?' is the definition.
(erosion leads to something crumbling away)
'not entirely zero defects' is the wordplay.
'not entirely' says the answer is hidden in the clue.
'ERODE' is hidden in the letters of 'zero defects'.
(Other definitions for erode that I've seen before include "Grind down" , "Become ground down or deteriorate" , "Abrade" , "Slowly remove" , "go to pieces!" .)