Order of Service is one for mass (9)
I believe the answer is:
programme
'order of service' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'one for mass' is the wordplay.
'one for' becomes 'pro' (as in being 'pro' or in favour of something).
'mass' becomes 'gramme' (unit of mass).
'pro'+'gramme'='PROGRAMME'
'is' is the link.
Can you help me to learn more?
(Other definitions for programme that I've seen before include "Schedule of events" , "Curriculum" , "Scheduled plan" , "Menu of events" , "Broadcast feature" .)