Calm witness takes in court briefly (6)
I believe the answer is:
sedate
'calm' is the definition.
('sedate' can be a synonym of 'calm')
'witness takes in court briefly' is the wordplay.
'witness' becomes 'see' (I've seen this before).
'takes in' means one lot of letters goes inside another.
'court' becomes 'date' (romantically pursue).
'briefly' means to remove the last letter.
'date' with its final letter removed is 'dat'.
'see' enclosing 'dat' is 'SEDATE'.
(Other definitions for sedate that I've seen before include "Staid and sober" , "Teased into being calm" , "grave" , "Dignified - staid" , "Calm, unhurried" .)