Particular way of doing something (5,8)
I believe the answer is:
modus operandi
'particular way of doing something' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for modus operandi that I've seen before include "Method of working" , "Particular way of doing something (Latin)" , "way of doing things" , "this shows how it's done" , "Classically typical style" .)