Plans returning this unwanted correspondence (4)
I believe the answer is:
spam
'this unwanted correspondence' is the definition.
(as in spam emails)
'plans returning' is the wordplay.
'plans' becomes 'maps' (I've seen this before).
'returning' is a reversal indicator.
'maps' backwards is 'SPAM'.
(Other definitions for spam that I've seen before include "Thoughtlessly send" , "Send unsolicited mail" , "Junk emails" , "(Send) junk mail" , "unwanted correspondence" .)