The most common and most likely answer to this crossword clue is the 4 letter word PASS
I've seen this crossword clue in The Daily Telegraph
P
A
S
S
More clues leading to the result PASS
- To go by hand
- What's needed for exam success? I can't answer that
- Having taken some hemp as smuggler, search me!
- I don't know what to do with the ball!
- Get through gap between peaks
- Overtake father and sons
- Route I don't know
- The old man’s second to overtake
- Fool’s under pressure to overtake
- Meet one’s maker when in outskirts of Paris
Buddy explains!

The straight part
We know that "way through mountains" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case way through mountains = pass.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 PASS mean?
PASS verb- To change place.
- To change in state or status
- To move through time.
- To be accepted.
- In any game, to decline to play in one's turn.
- To do or be better.
- To take heed.
- An opening, road, or track, available for passing; especially, one through or over some dangerous or otherwise impracticable barrier such as a mountain range; a passageway; a defile; a ford.
- A channel connecting a river or body of water to the sea, for example at the mouth (delta) of a river.
- A single movement, especially of a hand, at, over or along anything.
- A single passage of a tool over something, or of something over a tool.
- An attempt.
- Success in an examination or similar test.
- A thrust or push; an attempt to stab or strike an adversary.
- A thrust; a sally of wit.
- A sexual advance.
- The act of moving the ball or puck from one player to another.
- A passing of two trains in the same direction on a single track, when one is put into a siding to let the other overtake it.
- Permission or license to pass, or to go and come.
- A document granting permission to pass or to go and come; a passport; a ticket permitting free transit or admission
- An intentional walk.
- The act of overtaking; an overtaking manoeuvre.
- The state of things; condition; predicament; impasse.
- Estimation; character.
- (Chaucer) A part, a division. Compare passus.
- The area in a restaurant kitchen where the finished dishes are passed from the chefs to the waiting staff.
- An act of declining to play one's turn in a game, often by saying the word "pass".
- A run through a document as part of a translation, compilation or reformatting process.
- A password (especially one for a restricted-access website).