Pretentious person (5)
I believe the answer is:
pseud
'pretentious person' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pseud that I've seen before include "Socially pretentious person" , "Poser" , "Pretentious type" , "Poseur (colloq.)" , "Pretender - fraud" .)