Mob curtailed bad omen (4)
I believe the answer is:
crow
'bad omen' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'mob curtailed' is the wordplay.
'mob' becomes 'crowd' (I've seen this before).
'curtailed' means to remove the last letter.
'crowd' with its final letter removed is 'CROW'.
Can you help me to learn more?
(Other definitions for crow that I've seen before include "one often getting stoned!" , "Black bird; boast" , "Native American -- bird" , "bar" , "'As the - - flies'" .)