Posh medic beginning to use revolting language (4)
I believe the answer is:
urdu
'language' is the definition.
(Urdu is an example)
'posh medic beginning to use revolting' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'medic' becomes 'dr' (abbreviation for doctor).
'beginning to' indicates taking the first letters.
'revolting' is a reversal indicator.
The first letter of 'use' is 'u'.
'u'+'dr'+'u'='udru'
'udru' backwards is 'URDU'.
(Other definitions for urdu that I've seen before include "Asian language written in Arabic script" , "Language used in Pakistan, India" , "Language akin to Hindi" , "speech" , "Pakistani tongue" .)