It's a regular trade practice (6)
I believe the answer is:
custom
I believe this clue is a double definition.
'it's a regular trade' is the first definition.
(I've seen this before)
'practice' is the second definition.
(I know that custom is a type of practice)
(Other definitions for custom that I've seen before include "Habit, usage" , "Usual way of doing something" , "Local habit" , "Traditional usage" , "'Habit, practice (6)'" .)