The practice of patronage (6)
I believe the answer is:
custom
I believe this is a double definition.
'the practice' is the first definition.
(custom is a kind of practice)
'patronage' is the second definition.
(custom is a kind of patronage)
'of' is the link.
(Other definitions for custom that I've seen before include "Habit, usage" , "'Habit, practice (6)'" , "Tradition; habit" , "Way of doing things" , "Traditional usage" .)