Embellished lecture beginning to nark dons (6)
I believe the answer is:
ornate
'embellished' is the definition.
(synonyms)
'lecture beginning to nark dons' is the wordplay.
'lecture' becomes 'orate' (I've seen this before).
'beginning to' says to take the initial letters.
'dons' means one lot of letters goes inside another (don can mean to wear or be covered in).
The first letter of 'nark' is 'n'.
'orate' placed around 'n' is 'ORNATE'.
(Other definitions for ornate that I've seen before include "Highly decorative" , "(Of literary style) complex and with unusual words" , "Much-decorated" , "Richly or elaborately decorated" , "Lavishly decorated" .)