Calm tranquillity (8)
(Other definitions for serenity that I've seen before include "Calm composure, peace" , "Calmness" , "Peacefulness" , "Peace and quiet" , "Peace of mind" .)
This clue is a double definition.
'calm' is the first definition.
(being still is being calm)
'tranquillity' is the second definition.
(thesaurus)
(Other definitions for stillness that I've seen before include "Motionlessness" , "Lack of movement" , "Quiet, calm" , "Tranquil silence, calmness" , "Tells sins in motionless silence" .)