'Brill' old form of therapy? There's something wrong here (6)
I believe the answer is:
defect
'there's something wrong here' is the definition.
'brill old form of therapy?' is the wordplay.
'brill' becomes 'def' (both are slang terms meaning excellent).
'old form of therapy?' becomes 'ect' (abbreviation for electroconvulsive therapy).
'def'+'ect'='DEFECT'
(Other definitions for defect that I've seen before include "Imperfection - go over to the enemy" , "Blemish, fault" , "Failing - desert" , "Flaw - change sides" , "Deficiency" .)