Peacefulness (8)
I believe the answer is:
serenity
'peacefulness' is the definition.
(I know that peacefulness can be written as serenity)
This is the entire clue.
(Other definitions for serenity that I've seen before include "Tranquility, heartsease" , "Peace and quiet" , "Peace, tranquillity" , "The absence of mental stress or anxiety" , "Tiny seer in state of repose" .)