Plans on having junk mail sent back (4)
I believe the answer is:
maps
'plans' is the definition.
(I've seen this before)
'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' in reverse letter order is 'MAPS'.
'on having' is the link.
(Other definitions for maps that I've seen before include "Charts" , "Plans" , "Diagrams of geographical areas" , "Books of maps" , "Plots" .)