Sir Arthur's happiness (5)
I believe the answer is:
bliss
'happiness' is the definition.
(bliss is a kind of happiness)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bliss that I've seen before include "Intense happiness" , "Beatitude" , "Ecstasy; British composer" , "Extreme happiness" , "Highest happiness" .)