Calm or tranquil (6)
I believe the answer is:
placid
This clue is a double definition.
'calm' is the first definition.
(similar in meaning)
'tranquil' is the second definition.
(I know that tranquil can be written as placid)
(Other definitions for placid that I've seen before include "Collected" , "Peaceful and tranquil" , "Not easily upset or excited" , "Calm; equable" , "Tranquil, even-tempered" .)