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 there's a link between them I don't understand?
'lecture is about 100' is the wordplay.
'lecture' becomes 'class' (academic class).
'about' means one lot of letters goes inside another.
'100' becomes 'C' (Roman numeral).
'is' placed around '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" .)