Calm and collected (6)
'calm and collected' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sedate that I've seen before include "Calm, unhurried" , "Calm or spectral appartition" , "grave" , "Collected" , "Dignified and subdued" .)
(Other definitions for self-possessed that I've seen before include "In control of one's feelings" , "Composed, confident" .)