Faultless theory by student (5)
I believe the answer is:
ideal
'faultless' is the definition.
(I've seen this before)
'theory by student' is the wordplay.
'theory' becomes 'idea' (idea can mean a theory).
'by' means one lot of letters go next to another.
'student' becomes 'l' (as in L-plates for learner drivers).
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "Model of ethical behaviour" , "Perfect but not likely to become a reality" , "superb specimen" , "Embodying perfection" , "existing only in imagination" .)