Small instrument not in tune (5)
I believe the answer is:
sharp
'in tune' 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?
'small instrument' is the wordplay.
I cannot really see how this works, but
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is present in the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is found within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'not' is the link.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Punctually; raised note" , "Quick; pointed" , "Kind of musical note" , "Acidic; note" , "Acidic; smart and stylish" .)