The most common and most likely answer to this crossword clue is the 5 letter word TIBET
I've seen this crossword clue in Financial Times
T
I
B
E
T
More clues leading to the result TIBET
- Sip beer regularly in teetotal country
- Chinese region flyer has got to stay within
- Foot of Everest? I’m sure yeti lives here?
- Bird shelters exist in eastern highland region
- Eastern region relations coming over by punt
- State half-time wager
- I live in dry mountainous region
- Endlessly bitter about disputed land
- Disputed territory lay south of Thai borders
- Normancoming over ousts leader of Britons in this country
Buddy explains!

The straight part
We know that "mountainous region" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mountainous region = tibet.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.