Sit once more in tranquillity (6)
I believe the answer is:
repose
This clue is a double definition.
'sit once more' is the first definition.
('pose' can be a synonym for 'sit' so perhaps 'repose' is OK for 'sit once more')
'tranquillity' is the second definition.
(I know that tranquillity can be written as repose)
'in' is the link.
(Other definitions for repose that I've seen before include "take it easy" , "Rest - tranquillity" , "peace" , "place" , "Relaxation" .)