Follows the progress of junk mail sent back (4)
I believe the answer is:
maps
'follows the progress' is the definition.
I can't judge whether this defines the answer.
'junk mail sent back' is the wordplay.
'junk mail' becomes 'spam' (I've seen this before).
'sent back' shows that the letters should be reversed in order.
'spam' back-to-front is 'MAPS'.
'of' is the link.
Can you help me to learn more?
(Other definitions for maps that I've seen before include "Charts" , "Plans" , "Diagrams of geographical areas" , "Books of maps" , "Plots" .)