For a time, the musical instrument is sharp (5)
I believe the answer is:
thorn
'sharp' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'for a time the musical instrument' is the wordplay.
I cannot quite understand how this works, but
'time' could be 't' (abbreviation) and 't' is present in the answer.
'instrument' could be 'horn' (horn is a kind of instrument) and 'horn' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for thorn that I've seen before include "(Rose's) prickle" , "Sharp point; runic character" , "Sharp point on plant" , "pain in one's side" , "eg Prickle on rose" .)