Yearn to be in a posh farewell (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'yearn to be in a posh' is the wordplay.
'yearn' becomes 'die' (dying is a kind of yearning).
'to be in' means one lot of letters goes inside another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'a'+'u'='au'
'die' going within 'au' is 'ADIEU'.
(Other definitions for adieu that I've seen before include "Gallic goodbye" , "Goodbye, madame" , "So long abroad" , "Leave" , "Cheerio" .)