A student majority, nearly (6)
I believe the answer is:
almost
'nearly' is the definition.
(synonyms)
'a student majority' is the wordplay.
'student' becomes 'l' (as in L-plates for learner drivers).
'majority' becomes 'most' (most of a group).
'a'+'l'+'most'='ALMOST'
(Other definitions for almost that I've seen before include "Virtually" , "Nearly, not quite" , "Not entirely" , "Slightly short of" , "practically" .)