A posh girl originally organising sound reproduction (5)
I believe the answer is:
audio
'sound reproduction' is the definition.
(I've seen this before)
'a posh girl originally organising' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'girl' becomes 'di'.
'originally' says to take the initial letters.
The initial letter of 'organising' is 'o'.
'a'+'u'+'di'+'o'='AUDIO'
(Other definitions for audio that I've seen before include "Sound system" , "type of reproduction" , "Sound, as opposed to vision" , "Recording of acoustic signals" , "Prefix relating to sound" .)