Even-tempered (6)
I believe the answer is:
placid
'even-tempered' is the definition.
(I know that even-tempered can be written as placid)
This is all the clue.
(Other definitions for placid that I've seen before include "Peaceable and not easily disturbed" , "Quiet and still" , "Calm and not easily upset" , "Calm, peaceful" , "without ruffles" .)