Plato's porch (4)
I believe the answer is:
stoa
'plato's porch' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stoa that I've seen before include "Colonnade, covered walkway" , "Venue for Greek philosophers" , "Portico or roofed colonnade" , "Ancient Greek portico" , "Greek portico or colonnade" .)