Love fine route followed, naturally (2,6)
I believe the answer is:
of course
'naturally' is the definition.
(I know that naturally can be written as of course)
'love fine route followed' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'fine' becomes 'f' (abbreviation).
'route' becomes 'course' (course is a kind of route).
'followed' is a charade indicator (letters next to each other).
'o'+'f'+'course'='OF COURSE'
(Other definitions for of course that I've seen before include "Naturally, certainly" , "without question" , "Naturally, goes without saying" , "Certainly, without a doubt" , "Definitely" .)