Mother during rest returned message (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'mother during rest returned' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'during' indicates putting letters inside.
'rest' becomes 'lie' (synonyms).
'returned' shows that the letters should be reversed in order.
'lie' written backwards gives 'eil'.
'ma' placed inside 'eil' is 'EMAIL'.
(Other definitions for email that I've seen before include "Message sent via computer" , "Send one online" , "Electronic message" , "Spam perhaps" , "Messages between computer terminals" .)