Junk mail's climbing the charts (4)
I believe the answer is:
maps
'charts' is the definition.
('map' can be a synonym of 'chart')
'junk mail's climbing' is the wordplay.
'junk mail' becomes 'spam' (I've seen this before).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'spam' written backwards gives 'MAPS'.
'the' is the link.
(Other definitions for maps that I've seen before include "Plans" , "Diagrams of geographical areas" , "Charts" , "Plots" , "Books of maps" .)