Plans to send back this unwanted message (4)
I believe the answer is:
spam
'unwanted message' is the definition.
(unwanted email)
'plans to send back' is the wordplay.
'plans' becomes 'maps' (I've seen this in another clue).
'to send back' is a reversal indicator (I've seen 'sends back' mean this).
'maps' written backwards gives 'SPAM'.
'this' is the link.
(Other definitions for spam that I've seen before include "canned meat" , "Tinned processed meat" , "unwanted correspondence" , "luncheon meat" , "Uninvited email" .)