I am quick, posh and unrehearsed (9)
I believe the answer is:
impromptu
'unrehearsed' is the definition.
(I've seen this before)
'i am quick posh' is the wordplay.
'i am' becomes 'i'm'.
'quick' becomes 'prompt' (synonyms).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'im'+'prompt'+'u'='IMPROMPTU'
'and' acts as a link.
(Other definitions for impromptu that I've seen before include "Without pre-planning" , "Unplanned; instrumental piece" , "Unplanned; composition" , "Ad hoc: spontaneous-sounding musical piece mastered by Schubert" , "With little or no preparation or forethought" .)