Plans to return this junk mail (4)
I believe the answer is:
spam
'this junk mail' is the definition.
(I've seen this before)
'plans to return' is the wordplay.
'plans' becomes 'maps' (I've seen this before).
'to return' shows that the letters should be reversed in order.
'maps' written backwards gives 'SPAM'.
(Other definitions for spam that I've seen before include "unwanted correspondence" , "unsolicited messages" , "Junk email" , "unwanted delivery" , "one doesn't want on computer" .)