Certain rule accepted by party for trial (4)
I believe the answer is:
demo
'certain rule' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'accepted by party for trial' is the wordplay.
'accepted by' means one lot of letters goes inside another.
'party' becomes 'do' (do is a kind of party).
'for trial' becomes 'em' (I can't explain this - if you can you should give a lot more credence to this answer).
'do' placed around 'em' is 'DEMO'.
Can you help me to learn more?
(Other definitions for demo that I've seen before include "Brass instrument valve" , "Show" , "presentation" , "In brief, a public event as of protest under dome" , "Protest (abbr)" .)