Saw it's rum that's mixed (6)
I believe the answer is:
truism
'saw' 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?
'it's rum that's mixed' is the wordplay.
'that's mixed' is an anagram indicator.
'its'+'rum'='itsrum'
'itsrum' is an anagram of 'TRUISM'.
Can you help me to learn more?
(Other definitions for truism that I've seen before include "An abvious truth" , "Obviously correct and verified statement" , "An obvious statement that says noting new or interesting" , "self-evident statement" , "Self-evident saying" .)