It goes without saying (5)
I believe the answer is:
axiom
'it goes without saying' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for axiom that I've seen before include "Universally accepted proposition" , "Generally accepted truism" , "that goes without saying" , "Self-evident principle" , "A saying that's widely accepted as true" .)