Commercial on independent group of countries is so long (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'commercial on independent group of countries' is the wordplay.
'commercial' becomes 'ad'.
'on' is a charade indicator (letters next to each other).
'independent' becomes 'i'.
'group of countries' becomes 'eu'.
'ad'+'i'+'eu'='ADIEU'
'is' is the link.
(Other definitions for adieu that I've seen before include "farewell message" , "Goodbye in France" , "All the best" , "cheers" , "Gallic goodbye" .)