The low-down exasperate posher folk (6)
I believe the answer is:
gentry
'posher folk' is the definition.
'the low-down exasperate' is the wordplay.
'the low down' becomes 'gen' (I've seen this in other clues).
'exasperate' becomes 'try' (as in 'a trying situation').
'gen'+'try'='GENTRY'
(Other definitions for gentry that I've seen before include "Upper-crust people, just below nobility" , "people of breeding show" , "The landowning classes" , "cream?" , "Persons of high birth" .)