Lack of excellent treatment (6)
I believe the answer is:
defect
'lack' is the definition.
(I've seen this before)
'excellent treatment' is the wordplay.
'excellent' becomes 'def' (old-fashioned term for excellent).
'treatment' becomes 'ect' (ECT is an example).
'def'+'ect'='DEFECT'
'of' acts as a link.
(Other definitions for defect that I've seen before include "Flaw - change sides" , "Transfer allegiance to opposition" , "mistake" , "Go over to the enemy - it's a flaw" , "Fault - default" .)