Concerning residue selling (9)
I believe the answer is:
retailing
'selling' is the definition.
(I know that retailing is a type of selling)
'concerning residue' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'residue' becomes 'tailing' (I can't explain this - if you can you should believe this answer much more).
're'+'tailing'='RETAILING'
Can you help me to learn more?
(Other definitions for retailing that I've seen before include "shopkeeper's daily routine?" , "Commercial activity" , "Giving details" , "Selling (goods)" , "marketing" .)