Should be helpful, but it may be just rubbish! (3)
I believe the answer is:
tip
'rubbish' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tip that I've seen before include "single piece of asparagus, perhaps" , "offered at end of repast" , "Useful hint; end" , "Piece of useful advice" , "Lean over" .)