Singing one could be nasty when breathless (6)
I believe the answer is:
shanty
'singing' is the definition.
The definition and answer 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?
'one could be nasty when breathless' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'an' (I've seen this in other clues) and 'an' is found within the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for shanty that I've seen before include "Number at sea" , "Sailor's work song - ramshackle hut" , "Slum ghetto" , "shed" , "Sailors' song - poor dwelling" .)