About series of lessons, naturally (2,6)
I believe the answer is:
of course
'naturally' is the definition.
('of course' can be a synonym of 'naturally')
'about series of lessons' is the wordplay.
'about' becomes 'of' (**).
'series of lessons' becomes 'course' (I've seen this before).
'of'+'course'='OF COURSE'
(Other definitions for of course that I've seen before include "Certainly, without a doubt" , "Naturally, certainly" , "'Naturally, as might be expected (2,6)'" , "need I say" , "Definitely" .)