Down under a student grant (5)
I believe the answer is:
allow
'grant' is the definition.
('allow' can be a synonym of 'grant')
'down under a student' is the wordplay.
'down' becomes 'low' ('low' can be a synonym of 'down').
'under' is a charade indicator (letters next to each other).
'student' becomes 'l' (as in L-plates for learner drivers).
'a'+'l'='al'
'low' after 'al' is 'ALLOW'.
(Other definitions for allow that I've seen before include "Acknowledge - concede" , "Give leave to" , "Permit, let happen" , "Permit or concede" , "praise no more" .)