Class that's not so much working (6)
I believe the answer is:
lesson
'class' is the definition.
(I've seen this before)
'not so much working' is the wordplay.
'not so much' becomes 'less' (I've seen this before).
'working' becomes 'on'.
'less'+'on'='LESSON'
'that's' acts as a link.
(Other definitions for lesson that I've seen before include "Sermon or teaching session" , "Class session or reading from Bible" , "Severe reproof" , "Teaching period" , "Portion of the scriptures read in church" .)