Commercial that is posh, so long (5)
I believe the answer is:
adieu
'so long' is the definition.
(both can mean goodbye)
'commercial that is posh' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'ad'+'ie'+'u'='ADIEU'
(Other definitions for adieu that I've seen before include "Bye" , "Goodbye, monsieur" , "the last word in France" , "cheers" , "Goodbye, madame" .)