Weakness and evil in Chelsea he condemned (8,4)
I believe the answer is:
achilles heel
'weakness' is the definition.
(I've seen this before)
'evil in chelsea he condemned' is the wordplay.
'evil' becomes 'ill' (as in doing people ill).
'in' means one lot of letters goes inside another.
'condemned' is an anagram indicator.
'chelsea'+'he'='chelseahe'
'chelseahe' is an anagram of 'achesheel'.
'ill' going inside 'achesheel' is 'ACHILLES' HEEL'.
'and' acts as a link.
(Other definitions for achilles heel that I've seen before include "Small but crucial weakness" , "Small but fatal weakness" , "A chink in the armour" , "One's weak point" , "Vulnerable part" .)