Message intercepted by William eventually turned up (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'intercepted by william eventually turned up' is the wordplay.
'EMAIL' is hidden back-to-front within the letters.
But, I am not sure how this is indicated.
Can you help me to learn more?
(Other definitions for email that I've seen before include "Messages between computer terminals" , "Post" , "Message sent via computer" , "Internet-borne communication" , "Messages via the internet" .)