Student covered in a great deal of stuff used by gardeners (5)
I believe the answer is:
mulch
'stuff used by gardeners' is the definition.
(a material to insulate soil)
'student covered in a great deal' is the wordplay.
'student' becomes 'l'.
'covered in' indicates putting letters inside.
'a great deal' becomes 'much' ('much' can be a synonym of 'a great deal'**).
'l' inserted inside 'much' is 'MULCH'.
'of' is the link.
(Other definitions for mulch that I've seen before include "Loose material spread by gardener to protect roots" , "Garden dressing" , "Treat with compost" , "Matter protecting soil from drying out" , "that is good for the garden" .)