Speak pompously about knight decorated (6)
I believe the answer is:
ornate
'decorated' is the definition.
(similar in meaning)
'speak pompously about knight' is the wordplay.
'speak pompously' becomes 'orate' (to orate is to give a speech).
'about' means one lot of letters goes inside another.
'knight' becomes 'N' (chess abbreviation).
'orate' placed around 'n' is 'ORNATE'.
(Other definitions for ornate that I've seen before include "Heavily decorated" , "Highly decorated" , "Elaborate, flowery" , "Lavishly decorated" , "Over-elaborate" .)