Becomes relaxed under student charters (6)
I believe the answer is:
leases
'charters' is the definition.
('lease' can be a synonym of 'charter')
'becomes relaxed under student' is the wordplay.
'becomes relaxed' becomes 'eases'.
'under' says to put letters next to each other.
'student' becomes 'l' (as in L-plates for learner drivers).
'eases' after 'l' is 'LEASES'.
(Other definitions for leases that I've seen before include "Rents out" , "Enters into contract to use, say, land" .)