Second instrument not in tune? (5)
I believe the answer is:
sharp
'tune?' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'second instrument not' is the wordplay.
I cannot quite see how this works, but
'second' could be 's' (common abbreviation) and 's' is present in the answer.
'instrument' could be 'harp' (harp is a kind of instrument) and 'harp' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for sharp that I've seen before include "Keen - exactly" , "Cutting; note" , "exactly on time" , "Quick-witted - keen" , "Clearly defined (image)" .)