Unwanted messages get plans backwards (4)
I believe the answer is:
'unwanted messages' is the definition.
'plans backwards' is the wordplay.
'plans' becomes 'maps' (I've seen this in another clue).
'backwards' says the letters should be written in reverse.
'maps' written backwards gives 'SPAM'.
'get' acts as a link.
(Other definitions for spam that I've seen before include "Junk emails" , "Unwanted e-mails" , "Possible item from menu" , "email messages" , "canned meat" .)