One trying to make an impression so pure? Rubbish! (6)
I believe the answer is:
poseur
'one trying to make an impression' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'so pure? rubbish' is the wordplay.
'rubbish' indicates an anagram (a poor quality or rubbish spelling of the letters).
'so'+'pure'='sopure'
'sopure' anagrammed gives 'POSEUR'.
Can you help me to learn more?
(Other definitions for poseur that I've seen before include "Exhibitionist" , "One of hypocritical affectation" , "One trying to impress others" , "Uprose (anag.)" , "One who strikes attitudes, a phoney" .)