Communication from mother cutting fiction, rejected (5)
I believe the answer is:
'communication' is the definition.
(email is a kind of communication)
'mother cutting fiction rejected' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'fiction' becomes 'lie' (I've seen this in other clues).
'rejected' says the letters should be written in reverse.
'lie' back-to-front is 'eil'.
'ma' inserted within 'eil' is 'EMAIL'.
'from' acts as a link.
(Other definitions for email that I've seen before include "Spam perhaps" , "Electronic communication" , "Send one online" , "Message sent via computer" , "Post" .)