Not doing as much in class (6)
I believe the answer is:
lesson
'much in class?' is the definition.
'lesson' can be an answer for 'class?' (I've seen this before). I am not certain of the 'much in' bit.
'not doing' is the wordplay.
'not' becomes 'less' (I've seen this in other clues).
'doing' becomes 'on' (I can't justify this - if you can you should give a lot more credence to this answer).
'less'+'on'='LESSON'
'as' acts as a link.
(Other definitions for lesson that I've seen before include "Spell of instruction" , "Instructive experience" , "Unit of instruction" , "Reading or exercise for a pupil" , "Teaching session or reading from Bible" .)