Ancient portico (4)
I believe the answer is:
stoa
'ancient portico' 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 "Make a Greek porch out of oats" , "Oast (anag.)" , "Covered walk with colonnade" , "In Athens some columns" , "Series of columns covered" .)