Class's top student, always brainy (6)
I believe the answer is:
clever
'brainy' is the definition.
(I've seen this before)
'class's top student always' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
'student' becomes 'L' (as in L-plates for learner drivers).
'always' becomes 'ever' (synonyms).
The first letter of 'class' is 'c'.
'c'+'l'+'ever'='CLEVER'
(Other definitions for clever that I've seen before include "brilliant" , "Quick at learning and understanding" , "Bright - ingenious" , "Sharp-witted, intelligent" , "Bright; smart" .)