The most common and most likely answer to this crossword clue is the 4 letter word CITY
C
I
T
Y
More clues leading to the result CITY
- Catty set I abandoned in Rome once
- Coventry, for example — it replaces its centre
- Large settlement requires wisdom to discount substantial account
- Reproduction involves sex in place of work in the financial district
- Team of tummy ticklers heading north
- Nicosia? It is in Cyprus
- Charlotte, for instance, is extremely coy about exactly what’s needed
- Function not covering a large town
- Wells’ classification, maybe only half capacity?
- Civility depleted at the core in business community
Buddy explains!

The straight part
We know that "Rome once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rome once = city.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 CITY mean?
CITY noun- A large settlement, bigger than a town; sometimes with a specific legal definition, depending on the place.
- A settlement granted special status by royal charter or letters patent; traditionally, a settlement with a cathedral regardless of size.
- The central business district; downtown.
- A large amount of something (used after the noun).