Hardly an atheist’s farewell! (5)
I believe the answer is:
adieu
'farewell' is the definition.
(adieu is a kind of farewell)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "See you later" , "Farewell (French)" , "Farewell, Monsieur!" , "Fare thee well!" , "Goodbye to the French" .)