Brings to account characters found concealed by great hall in Athens (4)
I believe the answer is:
Here is my best explanation:
'concealed by great hall in athens' is the definition.
I can't judge whether this definition defines the answer.
'brings to account characters found' is the wordplay.
'found' says the answer is hidden in the clue.
'STOA' is hidden within 'brings to account characters'.
(Other definitions for stoa that I've seen before include "Venue for Greek philosophers", "Greek portico or colonnade", "Porch", "Covered walkway", "Covered walk with colonnade", "Portico or roofed colonnade", "classical colonnade", "Colonnade, covered walkway", "Architectural feature" and "series of columns".)