Message William expects to be partially rejected (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'william expects to be partially rejected' is the wordplay.
'to be partially' indicates a hidden word.
'rejected' is a reversal indicator.
'liame' is hidden in the letters of 'william expects'.
'liame' back-to-front is 'EMAIL'.
(Other definitions for email that I've seen before include "Spam perhaps" , "Send one online" , "Messages via the internet" , "Digital message" , "Electronic communication" .)