Certainly old, refocus after treatment (2,6)
I believe the answer is:
of course
'certainly' is the definition.
(I've seen this before)
'old refocus after treatment' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'after treatment' is an anagram indicator.
'refocus' with letters rearranged gives 'fcourse'.
'o'+'fcourse'='OF COURSE'
(Other definitions for of course that I've seen before include "Naturally, goes without saying" , "Definitely" , "Naturally, certainly" , "'Naturally, as might be expected (2,6)'" , "Obviously!" .)