Sontag maybe framing small criticism about once-sacred mountain (9)
I believe the answer is:
parnassus
'once-sacred mountain' is the definition.
(sacred mountain in Greece)
'sontag maybe framing small criticism about' is the wordplay.
'sontag maybe' becomes 'susan' (writer Susan Sontag).
'framing' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'criticism' becomes 'rap' (as in taking the rap for a mistake).
'about' says the letters should be written in reverse.
'susan' going around 's' is 'sussan'.
'sussan'+'rap'='sussanrap'
'sussanrap' in reverse letter order is 'PARNASSUS'.
(Other definitions for parnassus that I've seen before include "the height of poetry" , "Mountain where Delphi is located" , "Mountain sacred to Apollo and the Muses" , "Greek mountain sacred to ancient Muses" , "Mount sacred to the Muses" .)