The most common and most likely answer to this crossword clue is the 6 plus 4 letter word KHYBER PASS
K
H
Y
B
E
R
P
A
S
S
More clues leading to the result KHYBER PASS
Buddy explains!
The straight part
We know that "important part of old trade route" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case important part of old trade route = khyber 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.