Extra brilliant! (5)
I believe the answer is:
super
I believe this is a double definition.
'extra' is the first definition.
(thesaurus)
'brilliant' is the second definition.
(I've seen this before)
(Other definitions for super that I've seen before include "Prefix meaning above or informally great" , "senior cop" , "Prefix meaning over or extremely" , "Exceptionally good" , "Gushily great, excellent" .)