The most common and most likely answer to this crossword clue is the 6 letter word METROS
M
E
T
R
O
S
Buddy explains!
The straight part
We know that "Subway systems" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subway systems = metros.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 METROS mean?
METROS 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