Sir Arthur's great happiness (5)
I believe the answer is:
bliss
I believe this clue is a double definition.
'sir arthur's' is the first definition.
(as in the composer Sir Arthur Bliss, 1891-1975)
'great happiness' is the second definition.
(I've seen this before)
(Other definitions for bliss that I've seen before include "Seventh 13 across" , "Great happiness" , "Rabbit" , "Extreme happiness" , "Ecstasy; British composer" .)