An examination in a chamber (6)
I believe the answer is:
atrial
'in a chamber' is the definition.
(adjectival form of atrium/atria)
'an examination' is the wordplay.
'an' becomes 'a'.
'examination' becomes 'trial' (I've seen this before).
'a'+'trial'='ATRIAL'
(Other definitions for atrial that I've seen before include "Lariat (anag)" , "Like some courts" , "referring to Chambers?" , "of the heart cavities" , "Like main court" .)