Naturally it's to do with a series of lectures (2,6)
I believe the answer is:
of course
'naturally' is the definition.
(I know that naturally can be written as of course)
'of lectures' is the wordplay.
'lectures' becomes 'course' (a course can be a series of lectures).
'of'+'course'='OF COURSE'
'it's to do with a series' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for of course that I've seen before include "without question" , "Without a doubt, as was to be expected" , "Certainly, without a doubt" , "About a series of lessons" , "Definitely" .)