Intelligent? Brilliant! (6)
I believe the answer is:
bright
I believe this clue is a double definition.
'intelligent?' is the first definition.
('bright' can be similar in meaning to 'intelligent')
'brilliant' is the second definition.
(similar in meaning)
(Other definitions for bright that I've seen before include "Sparkling" , "Clever - shining - sunny" , "Intelligent - glaring" , "Blinding - intelligent" , "Clever; fine" .)