Spooner's crude technique that's used to send messages (5,4)
I believe the answer is:
morse code
'that's used to send messages' is the definition.
(code for sending messages)
I can't explain the remainder of the clue.
(Other definitions for morse code that I've seen before include "Means to communicate" , "Means of communication using dots and dashes" , "Means of signalling by dots and dashes" , "Dot and dash signals" , "Dot-dash signalling" .)