Tell stories about entertaining old lady's message (5)
I believe the answer is:
'message' is the definition.
(email is a type of message)
'tell stories about entertaining old lady's' is the wordplay.
'tell stories' becomes 'lie' (to tell stories is to lie or fib).
'about' is a reversal indicator.
'entertaining' means one lot of letters goes inside another (inviting in, as for dinner).
'old lady' becomes 'ma' (both can informally mean mother).
'lie' in reverse letter order is 'eil'.
'eil' placed around 'ma' is 'EMAIL'.
(Other definitions for email that I've seen before include "Internet-borne communication" , "Send one online" , "Digital message" , "Spam perhaps" , "Messages via the internet" .)