The most common and most likely answer to this crossword clue is the 5 letter word OSTIA
I've seen this crossword clue in The Times
O
S
T
I
A
More clues leading to the result OSTIA
- East German capital erected in Roman port
- It thus set up a Roman port
- Coming up, island like this old port
- Seaman comes to island, back in old harbour
- Old street, one area in ancient Italian port
- Old way one originally approached a Roman port
- Island thus seen looking west from Roman harbour
- East German I asked originally to identify old Rome’s port
- Seen from behind in portrait, some Roman port
- Country said to to drop right to Roman port
Buddy explains!

The straight part
We know that "Roman port" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case roman port = ostia.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OSTIA mean?
OSTIA noun- A small opening or orifice, as in a body organ or passage.
- Any of the small openings or pores in a sponge.
- The mouth of a river.