So long a day that is universal (5)
I believe the answer is:
adieu
'so long' is the definition.
('adieu' can be a synonym of 'so long')
'a day that is universal' is the wordplay.
'a day' becomes 'ad' (I've seen this in other clues**).
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'universal' becomes 'u'.
'ad'+'ie'+'u'='ADIEU'
(Other definitions for adieu that I've seen before include "Leave" , "See you later" , "Adios, Arrivederci" , "the last word in Paris" , "Goodbye, mon ami!" .)