Test on area of the chamber (6)
I believe the answer is:
atrial
'of the chamber' is the definition.
(relating to the atrium in the heart)
'test on area' is the wordplay.
'test' becomes 'trial' (synonyms).
'on' says to put letters next to each other.
'area' becomes 'a' (maths abbreviation).
'trial' after 'a' is 'ATRIAL'.
(Other definitions for atrial that I've seen before include "of the heart cavities" , "referring to Chambers?" , "Like main court" , "Lariat (anag)" , "Like some courts" .)