Junk mail returned the charts (4)
I believe the answer is:
maps
'charts' is the definition.
('map' can be a synonym of 'chart')
'junk mail returned' is the wordplay.
'junk mail' becomes 'spam' (I've seen this before).
'returned' says the letters should be written in reverse.
'spam' in reverse letter order is 'MAPS'.
'the' is the link.
(Other definitions for maps that I've seen before include "Books of maps" , "Diagrams of geographical areas" , "Plans" , "Plots" , "Charts" .)