Theoretical concept supported by student (5)
I believe the answer is:
ideal
'theoretical' is the definition.
(thesaurus)
'concept supported by student' is the wordplay.
'concept' becomes 'idea' (I've seen this before**).
'supported by' is a charade indicator (letters next to each other) (letters go below - in down clue).
'student' becomes 'l' (as in L-plates for learner drivers).
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "Perfect; a standard of perfection" , "Best possible" , "principle to be aimed at" , "Perfect; visionary" , "Perfect for the purpose" .)