One hanging poster about product’s unmatched quality (9)
I believe the answer is:
suspender
'one hanging' is the definition.
(something or someone suspended or hanging)
'poster about product's unmatched quality' is the wordplay.
'poster' becomes 'sender' (somebody sending or posting something).
'about' means one lot of letters goes inside another.
'product's unmatched quality' becomes 'USP' (unique selling point).
'sender' enclosing 'usp' is 'SUSPENDER'.
(Other definitions for suspender that I've seen before include "material support" , "One that supports stocking" , "Stocking-holding strap" , "hangman?" , "hanger on?" .)