Citrus fruit returned without a message (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'citrus fruit returned without a' is the wordplay.
'citrus fruit' becomes 'lime' (lime is a kind of citrus fruit).
'returned' says the letters should be written in reverse.
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'lime' written backwards gives 'emil'.
'emil' enclosing 'a' is 'EMAIL'.
(Other definitions for email that I've seen before include "Spam perhaps" , "Digital message" , "Send one online" , "Electronic communication" , "Electronic message" .)