Lecture is about 100 great books (8)
I believe the answer is:
classics
'great books' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Maybe you can see a link between them that I don't see?
'lecture is about 100' is the wordplay.
'lecture' becomes 'class' (academic class).
'about' is an insertion indicator.
'100' becomes 'C' (Roman numeral).
'is' enclosing 'c' is 'ics'.
'class'+'ics'='CLASSICS'
Can you help me to learn more?
(Other definitions for classics that I've seen before include "The study of ancient Greek and Roman culture" , "Course" , "horse races" , "Literary works of ancient Greece and Rome" , "Long established and enduring works of art" .)