Not so much on part of the school timetable (6)
I believe the answer is:
lesson
'the school timetable' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'not so much on' is the wordplay.
'not so much' becomes 'less' (I've seen this in another clue).
'less'+'on'='LESSON'
'part of' acts as a link.
Can you help me to learn more?
(Other definitions for lesson that I've seen before include "Portion of instruction" , "15 down period" , "Piece of instruction" , "Class session or reading from Bible" , "Sermon or teaching session" .)