Not back at school a great deal (4)
I believe the answer is:
tons
'a great deal' is the definition.
(both can mean a large amount)
'not back at school' is the wordplay.
'back' suggests the final letters (the letter at the back).
'at' becomes 'on' (location).
'school' becomes 'S'.
The final letter of 'not' is 't'.
't'+'on'+'s'='TONS'
(Other definitions for tons that I've seen before include "A lot" , "Lots" , "Each is equivalent to 2240 pounds" , "Loads (sl.)" .)