Retiring saint on ecstasy ruled a bad practitioner (7)
I believe the answer is:
shyster
'practitioner' is the definition.
(I have seen 'Unscrupulous practitioner' mean 'shyster' so perhaps 'practitioner' could also mean 'shyster')
'retiring saint on ecstasy ruled a bad' is the wordplay.
'retiring' becomes 'shy' (I've seen this in other clues).
'saint' becomes 'st'.
'on' means one lot of letters go next to another.
'ecstasy' becomes 'E' (informal term for the drug).
'ruled a bad' becomes 'r' (I can't explain this - if you can you should give a lot more credence to this answer).
'shy'+'st'+'e'+'r'='SHYSTER'
Can you help me to learn more?
(Other definitions for shyster that I've seen before include "Unscrupulous lawyer" , "Disreputable practitioner (especially lawyer)" , "Dodgy lawyer" , "Crooked bar person" , "Lawyer using unethical methods" .)