Seems a girl regularly used communication system (5)
I believe the answer is:
'communication system' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'seems a girl regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'seems'+'a'+'girl'='seemsagirl'
The alternate letters of 'seemsagirl' are 'EMAIL'.
'used' is the link.
Can you help me to learn more?
(Other definitions for email that I've seen before include "Messages via the internet" , "Send one online" , "Internet-borne communication" , "Messages between computer terminals" , "Electronic communication" .)