Fancy lecture about Newton (6)
I believe the answer is:
ornate
'fancy' is the definition.
(thesaurus)
'lecture about newton' is the wordplay.
'lecture' becomes 'orate' (I've seen this before).
'about' is an insertion indicator.
'newton' becomes 'n' ('N' can be a synonym of 'newton').
'orate' placed around 'n' is 'ORNATE'.
(Other definitions for ornate that I've seen before include "Ostentatious" , "Rich in decorative detail" , "Highly embellished" , "Heavily decorated" , "Richly adorned and elaborate" .)