Chamber of the heart (6)
I believe the answer is:
atrium
'chamber of the heart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atrium that I've seen before include "Roofed courtyard" , "Central court/hall" , "Central hall or court" , "Um, I rat on the chamber of the heart" , "Central court of ancient Roman house" .)