Seller with express service (6)
I believe the answer is:
repair
'service' is the definition.
(I've seen this before)
'seller with express' is the wordplay.
'seller' becomes 'rep' (sales rep).
'with' says to put letters next to each other.
'express' becomes 'air' (I've seen this before).
'rep'+'air'='REPAIR'
(Other definitions for repair that I've seen before include "Restore to working order" , "Restore to good order" , "Proceed - restore" , "Correct" , "Mend, restore to good condition" .)