Suffer a very good student (3)
I believe the answer is:
ail
'suffer' is the definition.
(ailing is a kind of suffering)
'a very good student' is the wordplay.
'a very good' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'student' becomes 'l' (as in L-plates for learner drivers).
'ai'+'l'='AIL'
(Other definitions for ail that I've seen before include "Sink" , "Become unwell" , "Be ill or unwell" , "Sicken" , "Be in poor health" .)