Long salesperson's talk lacking power (4)
I believe the answer is:
itch
'long' is the definition.
(to long or itch for something is to desire it)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for itch that I've seen before include "Hunger, longing" , "Tickling sensation of skin" , "Irritating sensation on skin" , "Rash reaction?" , "Longing; anag. of 19" .)