Message I put in flimsy cover returned (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'i put in flimsy cover returned' is the wordplay.
'put in' means one lot of letters goes inside another.
'flimsy cover' becomes 'lame' (type of fabric).
'returned' is a reversal indicator.
'i' put into 'lame' is 'liame'.
'liame' back-to-front is 'EMAIL'.
(Other definitions for email that I've seen before include "Post" , "Messages via the internet" , "Message sent via computer" , "Electronic communication" , "Electronic message" .)