Catholic student, always smart (6)
I believe the answer is:
clever
'smart' is the definition.
(similar in meaning)
'catholic student always' is the wordplay.
'catholic' becomes 'C' (abbreviation as in RC for Roman Catholic).
'student' becomes 'l' (as in L-plates for learner drivers).
'always' becomes 'ever' (synonyms).
'c'+'l'+'ever'='CLEVER'
(Other definitions for clever that I've seen before include "Knowledgeable" , "Quick-minded" , "Smart or shrewd" , "Quick, skilful" , "Quick at learning and understanding" .)