A judicial hearing in a main court (6)

I believe the answer is:

atrial

'in a main court' is the definition.
(in an atrium)

'a judicial hearing' is the wordplay.
'judicial hearing' becomes 'trial' (trial in court).
'a'+'trial'='ATRIAL'

(Other definitions for atrial that I've seen before include "Like some courts" , "referring to Chambers?" , "Like main court" , "Lariat (anag)" , "of the heart cavities" .)

I've seen this clue in The Times.

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