The most common and most likely answer to this crossword clue is the 3 letter word ALP
I've seen this crossword clue in The Times
A
L
P
Buddy explains!
The straight part
We know that "High mountain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case high mountain = alp.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 ALP mean?
ALP noun- A very high mountain. Specifically, one of the Alps, the highest chain of mountains in Europe.