Calm, tranquil (6)
I believe the answer is:
placid
I believe this clue is a double definition.
'calm' is the first definition.
(synonyms)
'tranquil' is the second definition.
(I know that tranquil can be written as placid)
(Other definitions for placid that I've seen before include "mild" , "Calm and not easily upset" , "Quiet and still" , "Not ruffled" , "Calm, peaceful" .)