Class taking in posh meeting (5)
I believe the answer is:
forum
'meeting' is the definition.
(forum is a kind of meeting)
'class taking in posh' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'taking in' indicates putting letters inside.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'form' placed around 'u' is 'FORUM'.
(Other definitions for forum that I've seen before include "Place of assembly for the people in ancient Greece" , "A meeting to exchange views" , "Place where public discussion is held, as in ancient Rome" , "Meeting of people for discussion" , "Internet message board" .)