Excellent student to sicken (3)
I believe the answer is:
ail
'sicken' is the definition.
(I've seen this in another clue)
'excellent student' is the wordplay.
'excellent' becomes 'ai' (resembles 'Al', old-fashioned term for excellent).
'student' becomes 'l' (as in L-plates for learner drivers).
'ai'+'l'='AIL'
'to' acts as a link.
(Other definitions for ail that I've seen before include "upset" , "Be unwell, feel poorly" , "Be the worse for wear" , "Go sick" , "Bother" .)