Delta that is in Australia and so long (5)
I believe the answer is:
adieu
'so long' is the definition.
(both can mean goodbye)
'delta that is in australia' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'in' means one lot of letters goes inside another.
'australia' becomes 'au'.
'd'+'ie'='die'
'die' put into 'au' is 'ADIEU'.
'and' is the link.
(Other definitions for adieu that I've seen before include "Goodbye, monsieur" , "See you later" , "Fare thee well!" , "Parting word" , "Vale" .)