The most common and most likely answer to this crossword clue is the 6 letter word ISTHMI
I've seen this crossword clue in The Guardian
I
S
T
H
M
I
More clues leading to the result ISTHMI
- For starters, I’ll serve this halloumi marinated in strips
- Saint and queen accessing two narrow areas
- They cross water using this motorway
- Connecting areas, this diverse motorway
- After the last dreg of martini, this man, getting intoxicated, strips in water
- After commencement of inflammation, Smith is worried about constrictions
- . I'm confused about this badly! Necks of lands?
- Bands played “It’s him”
- For starters, I'll serve this halloumi marinated in strips
- Pitching, mishit necks?
Buddy explains!

The straight part
We know that "narrow areas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case narrow areas = isthmi.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 ISTHMI mean?
ISTHMI noun- A narrow strip of land, bordered on both sides by water, and connecting two larger landmasses.
- Any such narrow part connecting two larger structures.
- An edge in a graph whose deletion increases the number of connected components of the graph.