Most recent top grade is after student examination (6)
I believe the answer is:
latest
'most recent' is the definition.
('late' can be similar in meaning to 'recent')
'top grade is after student examination' is the wordplay.
'top grade' becomes 'a'.
'is after' says to put letters next to each other.
'student' becomes 'l' (as in L-plates for learner drivers).
'examination' becomes 'test' (synonyms).
'a' after 'l' is 'la'.
'la'+'test'='LATEST'
(Other definitions for latest that I've seen before include "Brand new, just out" , "hot bit of news" , "State-of-the-art" , "Most recent or tardiest" , "Stop press news?" .)