Flier finished crossing river (5)
I believe the answer is:
drone
'flier' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'finished crossing river' is the wordplay.
'finished' becomes 'done' (similar in meaning).
'crossing' indicates putting letters inside.
'river' becomes 'r'.
'done' going around 'r' is 'DRONE'.
Can you help me to learn more?
(Other definitions for drone that I've seen before include "he gives us a buzz" , "Continuous hum" , "Continuous sound; bee" , "Bertie Wooster, say" , "Sustained bass note accompanying a melody - male bee" .)