Rapid communication could be a great help (9)
I believe the answer is:
telegraph
'rapid communication' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'could be a great help' is the wordplay.
'could be' indicates anagramming the letters.
'great'+'help'='greathelp'
'greathelp' with letters rearranged gives 'TELEGRAPH'.
Can you help me to learn more?
(Other definitions for telegraph that I've seen before include "Signal" , "Transmission (by wire on poles)" , "System of communication over wires" , "It's used for communications over wires" , "A scoreboard" .)