It's traumatic losing cat from open courtyard (6)
I believe the answer is:
atrium
'courtyard' is the definition.
(atrium is a kind of courtyard)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for atrium that I've seen before include "Entrance hall" , "lobby" , "Central court/hall" , "Central hall or court" , "Central area in a building, open or glass-roofed" .)