Rest, tranquillity (6)
I believe the answer is:
repose
I believe this clue is a double definition.
'rest' is the first definition.
(I know that rest can be written as repose)
'tranquillity' is the second definition.
(I know that tranquillity can be written as repose)
(Other definitions for repose that I've seen before include "take it easy" , "Lie at rest" , "calm" , "Rest - tranquillity" , "place" .)