"Naturally" (2,6)
I believe the answer is:
of course
'naturally' is the definition.
(I know that naturally can be written as of course)
This is the entire clue.
(Other definitions for of course that I've seen before include "Without a doubt, as was to be expected" , "without question" , "Naturally, goes without saying" , "need I say" , "Naturally, as one would expect" .)