The type of tablets that were once crushed for prescriptions devised by the man upstairs (3,12)
I believe the answer is:
ten commandments
'the type of tablets that were' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'once crushed for prescriptions devised by the man upstairs' is the wordplay.
I cannot really understand how this works, but
an anagram of 'once' is 'enco' which is within the answer.
'man' could be 'n' (knight, a man in chess) and 'n' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for ten commandments that I've seen is " Orders from on high".)