Relative, terribly rude, shows splendour (8)
I believe the answer is:
grandeur
'splendour' is the definition.
(synonyms)
'relative terribly rude' is the wordplay.
'relative' becomes 'gran' (short for grandmother).
'terribly' is an anagram indicator.
'rude' with letters rearranged gives 'deur'.
'gran'+'deur'='GRANDEUR'
'shows' acts as a link.
(Other definitions for grandeur that I've seen before include "Nobility, dignity" , "Magnificence - rare dung (anag)" , "Augustness" , "Impressiveness of appearance" , "Loftiness" .)