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.
Perhaps they are linked in a way I don't understand?
'for a time the musical instrument' is the wordplay.
I cannot really understand how this works, but
'time' could be 't' (abbreviation) and 't' is located 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' is the 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" .)