Crossword Genius

At the front of the building strong wine could be served before Irish Chamber Orchestra appears (7)

Ross

I believe the answer is:

portico

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'at' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for portico that I've seen before include "gallery, perhaps" , "Covered walkway" , "Grand entrance" , "Colonnade, covered porch" , "small architectural feature" .)

I've seen this clue in The Irish Times.
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