Proud US rival I go on about (12)
I believe the answer is:
vainglorious
'proud' is the definition.
('vainglorious' can be similar in meaning to 'proud')
'us rival i go on about' is the wordplay.
'about' indicates an anagram.
'us'+'rival'+'i'+'go'+'on'='usrivaligoon'
'usrivaligoon' anagrammed gives 'VAINGLORIOUS'.
(Other definitions for vainglorious that I've seen before include "Cocky" , "Self-important -- savouring oil (anag)" , "Boasting" , "Boastful; rousing viola (anag.)" , "Excessively self-important" .)