Weakness that’s not hard to see (4,4)
I believe the answer is:
soft spot
'weakness' is the definition.
(I've seen this before)
'not hard to see' is the wordplay.
'not hard' becomes 'soft' ('soft' is the opposite to 'hard').
'to see' becomes 'spot' (I've seen this before).
'soft'+'spot'='SOFT SPOT'
'that's' is the link.
(Other definitions for soft spot that I've seen before include "Affectionate weakness for (someone/something)" , "Sentimental affection" , "Emotional susceptibility" , "Sentimental fondness" , "Feeling of affection towards person - not hard" .)