Weakness, vulnerable point (8,4)
I believe the answer is:
achilles heel
I believe this clue is a double definition.
'weakness' is the first definition.
(I've seen this before)
'vulnerable point' is the second definition.
(I've seen this before)
(Other definitions for achilles heel that I've seen before include "vulnerability" , "Personal weak spot" , "A chink in the armour" , "Person's vulnerable weakness" , "One's most vulnerable spot" .)