Left one firm architectural feature (7)

I believe the answer is:

portico

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'architectural feature' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'left one firm' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'one' becomes 'i' (Roman numeral).
'firm' becomes 'co' (abbreviation for company).
'port'+'i'+'co'='PORTICO'

Can you help me to learn more?

(Other definitions for portico that I've seen before include "Covered porch with supporting columns" , "Arcade" , "A number of columns" , "Colonnade in front of a building" , "Classical porch" .)

I've seen this clue in The Daily Express.

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