Goodbye to commercial that is high class (5)
I believe the answer is:
adieu
'goodbye' is the definition.
(synonyms)
'commercial that is high class' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'high class' becomes 'U' (as in U and non-U speech).
'ad'+'ie'+'u'='ADIEU'
'to' is the link.
(Other definitions for adieu that I've seen before include "Vale" , "I'm going" , "Parting greeting" , "the last word in France" , "left, making this?" .)