Regularly pre-empt one trying to sell goods (3)
I believe the answer is:
rep
'one trying to sell goods' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'regularly pre-empt' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'pre'+'empt'='preempt'
The alternate letters of 'preempt' are 'REP'.
Can you help me to learn more?
(Other definitions for rep that I've seen before include "Travelling salesman" , "Representative; fabric" , "agent" , "Theatre; fabric; salesman" , "Corded cloth for short theatrical group" .)