Achilles' heel (4,4)
I believe the answer is:
weak spot
'achilles heel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for weak spot that I've seen before include "Point of vulnerability" , "Achilles heel?" , "Area in which one is not knowledgeable" , "Gap in knowledge" , "'Your area of vulnerability (4,4)'" .)