The most common and most likely answer to this crossword clue is the 5 letter word METRO
M
E
T
R
O
More clues leading to the result METRO
- Transport system satisfied staff will need day off
- Underground newspaper
- Sort of sexual underground?
- To surrender beat, The Jam finally retire underground
- Underground ring supporting police officer in the end
- Underground I’m better, not seen regularly
- New term for dispensing with borders in transport system
- Form of transport, note, linked with city of Paris mainly
- Ran across right over railway
- Transport in Paris or Rome, perhaps, around the West End
Buddy explains!

The straight part
We know that "transport system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case transport system = metro.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 METRO mean?
METRO noun- An underground railway.
- A train that runs on such an underground railway.
- An urban rapid transit light railway
- A train that runs on such a railway.
- A metropolitan area
- Metropolitan