Communication held back by parliament (5)
I believe the answer is:
'communication' is the definition.
(I know that email is a type of communication)
'held back by parliament' is the wordplay.
'back by' says the letters should be written backwards.
'held' indicates the answer is hidden within the clue.
'liame' is hidden in the letters of 'parliament'.
'liame' in reverse letter order is 'EMAIL'.
(Other definitions for email that I've seen before include "Spam perhaps" , "Messages via the internet" , "Electronic message" , "Message sent via computer" , "Internet-borne communication" .)