Happening to meet posh friend at last (10)
I believe the answer is:
eventually
'at last' is the definition.
'happening to meet posh friend' is the wordplay.
'happening' becomes 'event' (I've seen this before).
'to meet' is a charade indicator (letters next to each other).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'friend' becomes 'ally' (I've seen this before).
'event'+'u'+'ally'='EVENTUALLY'
(Other definitions for eventually that I've seen before include "sooner or later" , "In the end" , "In due course" , "At last" , "Finally" .)