Pretentious person rose up unexpectedly (6)
I believe the answer is:
poseur
'pretentious person' is the definition.
(I've seen this before)
'rose up unexpectedly' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'rose'+'up'='roseup'
'roseup' anagrammed gives 'POSEUR'.
(Other definitions for poseur that I've seen before include "Person habitually pretending to be something he is not" , "Person who strikes attitudes, a show-off" , "Pretentious chap" , "Affected person, strikes attitudes" , "Uprose (anag.)" .)