Plans found in junk mail returned (4)
I believe the answer is:
maps
'plans' is the definition.
(I know this)
'junk mail returned' is the wordplay.
'junk mail' becomes 'spam' (I've seen this before).
'returned' shows that the letters should be reversed in order.
'spam' backwards is 'MAPS'.
'found in' is the link.
(Other definitions for maps that I've seen before include "Diagrams of geographical areas" , "Plots" , "Books of maps" , "Charts" , "Plans" .)