Maps rise of unwanted email (4)
I believe the answer is:
spam
'unwanted email' is the definition.
(I've seen this before)
'maps rise' is the wordplay.
'rise' says the letters should be written in reverse.
'maps' in reverse letter order is 'SPAM'.
'of' is the link.
(Other definitions for spam that I've seen before include "Send unsolicited mail" , "Unwanted e-mails" , "Junk emails" , "Computer junk mail" , "unsolicited communication" .)