Sibyls unfortunately lost unknown happiness (5)
I believe the answer is:
bliss
'happiness' is the definition.
(bliss is a kind of happiness)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for bliss that I've seen before include "Extreme happiness" , "rhapsody?" , "British composer, Arthur (1881-1975) - perfect happiness" , "Highest happiness" , "Seventh 13 across" .)