Parts of hearts (5)
I believe the answer is:
atria
'parts of hearts' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atria that I've seen before include "Parts of the heart or of Roman houses" , "Central areas in buildings; open to the sky" , "Open courts of Roman houses" , "Central courts (in ancient Rome or shopping arcades)" , "Central courts, halls" .)