Brilliance of older puns lost (9)
I believe the answer is:
splendour
'brilliance' is the definition.
(I know that brilliance can be written as splendour)
'older puns lost' is the wordplay.
'lost' indicates anagramming the letters.
'older'+'puns'='olderpuns'
'olderpuns' is an anagram of 'SPLENDOUR'.
'of' is the link.
(Other definitions for splendour that I've seen before include "Magnificent appearance" , "show" , "Glory, brilliance" , "Spectacle" , "Opulence" .)