A cold fish accepting she shows weakness (8,4)
I believe the answer is:
achilles heel
'weakness' is the definition.
(I've seen this before)
'a cold fish accepting she' is the wordplay.
'cold' becomes 'chill' (chill is a kind of cold).
'fish' becomes 'eel' (eel is a kind of fish).
'accepting' is an insertion indicator.
'eel' going around 'she' is 'esheel'.
'a'+'chill'+'esheel'='ACHILLES' HEEL'
'shows' is the link.
(Other definitions for achilles heel that I've seen before include "Only vulnerable point" , "Small but fatal weakness" , "Weak spot for attack" , "flaw" , "Vulnerable part" .)