The most common and most likely answer to this crossword clue is the 5 plus 4 letter word NANDA DEVI
I've seen this crossword clue in The Times
N
A
N
D
A
D
E
V
I
More clues leading to the result NANDA DEVI
Buddy explains!

The straight part
We know that "a mountain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a mountain = nanda devi.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.