Portsmouth & Co (5)

I believe the answer is:

ostia

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'portsmouth co' is the definition.
The definition suggests a singular noun which matches the answer.

This is the entire clue.

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(Other definitions for ostia that I've seen before include "ancient town" , "ruined port" , "Ancient harbour" , "The old port of Rome" , "openings in Rome" .)

I've seen this clue in The Guardian.

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