Regularly supplies knight with serious grandeur (9)
I believe the answer is:
splendour
'grandeur' is the definition.
(I know that grandeur can be written as splendour)
'regularly supplies knight with serious' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'knight' becomes 'N' (chess abbreviation).
'with' says to put letters next to each other.
'serious' becomes 'dour' (I've seen this in other clues).
The alternate letters of 'supplies' are 'sple'.
'sple'+'n'+'dour'='SPLENDOUR'
(Other definitions for splendour that I've seen before include "Opulence" , "Brilliance or pomp" , "Brilliant appearance" , "Spectacle" , "Magnificence, brilliance" .)