Posh doctor living in modern flat (9)
I believe the answer is:
recumbent
'flat' is the definition.
(thesaurus)
'posh doctor living in modern' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'living in' is an insertion indicator.
'modern' becomes 'recent' (I've seen this before**).
'u'+'mb'='umb'
'umb' placed into 'recent' is 'RECUMBENT'.
(Other definitions for recumbent that I've seen before include "flat?" , "Lying down comfortably" .)