Weakness that’s easy to see (4,4)
I believe the answer is:
soft spot
'weakness' is the definition.
(thesaurus)
'easy to see' is the wordplay.
'easy' becomes 'soft' ('soft' can be similar in meaning to 'easy').
'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 "showing fondness" , "Sentimental fondness" , "Sentimental affection" , "Emotional susceptibility" , "Feeling of affection towards person - not hard" .)