Peaceful, tranquil (6)
I believe the answer is:
serene
This clue is a double definition.
'peaceful' is the first definition.
(similar in meaning)
'tranquil' is the second definition.
(I know that tranquil can be written as serene)
(Other definitions for serene that I've seen before include "Calm and unclouded" , "Not agitated" , "Totally unagitated" , "Peaceful and contented" , "still" .)