Medium greeting used for long-distance communication (5,4)
I believe the answer is:
radio wave
'long-distance communication' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'medium greeting' is the wordplay.
'medium' becomes 'radio' (I've seen this in another clue).
'greeting' becomes 'wave' (I've seen this before).
'radio'+'wave'='RADIO WAVE'
'used for' acts as a link.
Can you help me to learn more?
(Other definitions for radio wave that I've seen before include "Long-distance message-carrier" , "Carrier of sound" , "Transmission signal" .)