Serenity, tranquillity (5)
I believe the answer is:
peace
This clue is a double definition.
'serenity' is the first definition.
(I know that serenity can be written as peace)
'tranquillity' is the second definition.
(I know that peace is a type of tranquillity)
(Other definitions for peace that I've seen before include "Quiet companion" , "Calm, tranquility" , "Caught" , "still?" , "an answer to all clues, initially" .)