Pure credo that changes routine (9)
I believe the answer is:
procedure
'routine' is the definition.
(I know that routine can be written as procedure)
'pure credo that changes' is the wordplay.
'that changes' is an anagram indicator.
'pure'+'credo'='purecredo'
'purecredo' is an anagram of 'PROCEDURE'.
(Other definitions for procedure that I've seen before include "Sequence of steps, course of action" , "way to get things done" , "System for accomplishing something" , "Routine" , "Set sequence of events to achieve a result" .)