The most common and most likely answer to this crossword clue is the 6 plus 6 letter word SIERRA NEVADA
I've seen this crossword clue in Financial Times
S
I
E
R
R
A
N
E
V
A
D
A
More clues leading to the result SIERRA NEVADA
- Range area invaders ravaged
- Find range in destroying invader's area
- Venus oddly is back fully in range
- Rocky area with ravines around the back of beyond, as mountain range
- German you managed to avoid getting a replacement for electric range
- Queen in arena, diva’s perhaps showing range
- Invaders are prepared going over a range
- Ford a channel going west, following new range
Buddy explains!
The straight part
We know that "mountain range" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mountain range = sierra nevada.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.