Farewell to advert that is universal (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
'advert that is universal' is the wordplay.
'advert' becomes 'ad' (synonyms).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'universal' becomes 'U' (film certificate).
'ad'+'ie'+'u'='ADIEU'
'to' acts as a link.
(Other definitions for adieu that I've seen before include "farewell message" , "Goodbye, monsieur" , "A valediction" , "Parting word" , "I'm going" .)