Maps returned: unwanted mail (4)
I believe the answer is:
spam
'unwanted mail' is the definition.
(junk email)
'maps returned' is the wordplay.
'returned' shows that the letters should be reversed in order.
'maps' back-to-front is 'SPAM'.
(Other definitions for spam that I've seen before include "unwanted delivery" , "Send unsolicited mail" , "unwanted correspondence" , "canned meat" , "unsolicited communication" .)