... to pass out as part of a posh farewell (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'to pass out as part of a posh' is the wordplay.
'to pass out' becomes 'die'.
'as part of' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'a'+'u'='au'
'die' inserted inside 'au' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Farewell (French)" , "Adios, Arrivederci" , "Farewell, Monsieur!" , "Farewel" , "Leave" .)