Old relative was not happy about rising extravagance (8)
I believe the answer is:
grandeur
'extravagance' is the definition.
(grandeur is extravagance or majesty)
'old relative was not happy about rising' is the wordplay.
'old relative' becomes 'gran' (grandmother).
'was not happy about' becomes 'rued' (to rue something is to regret it).
'rising' says the letters should be written backwards (in a down clue, letters go up).
'rued' in reverse letter order is 'deur'.
'gran'+'deur'='GRANDEUR'
(Other definitions for grandeur that I've seen before include "Greatness, perhaps ironic" , "state" , "Impressive splendour" , "Splendour; high rank" , "Splendour, impressiveness" .)