Insincere show-off (6)
I believe the answer is:
poseur
'insincere show-off' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for poseur that I've seen before include "Hypocrite!" , "One who strikes attitudes, a phoney" , "Person who strikes attitudes, a show-off" , "Pretentious chap" , "Insincere show-off" .)