They're brilliant but I'm dense, confused about answer (5,3)

'they're brilliant' is the definition.
(ideas men are brilliant or innovative people)

'i'm dense confused about answer' is the wordplay.
'confused' indicates an anagram.
'about' indicates putting letters inside.
'answer' becomes 'a' (abbreviation eg in Q and A).
'im'+'dense'='imdense'
'imdense' anagrammed gives 'idesmen'.
'idesmen' placed around 'a' is 'IDEAS MEN'.

'but' acts as a link.