Good having confirmation of legality for indulgent pleasure (13)
I believe the answer is:
gratification
'indulgent pleasure' is the definition.
'good having confirmation of legality' is the wordplay.
'good' becomes 'g' (abbreviation).
'having' means one lot of letters go next to another.
'confirmation of legality' becomes 'ratification'.
'g'+'ratification'='GRATIFICATION'
'for' acts as a link.
(Other definitions for gratification that I've seen before include "Delight" , "Feeling of satisfaction" , "giving pleasure" , "Source of pleasure" .)