The most common and most likely answer to this crossword clue is the 6 letter word PRUSIK
P
R
U
S
I
K
Buddy explains!
The straight part
We know that "mountaineer’s sling" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mountaineer’s sling = prusik.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 PRUSIK mean?
PRUSIK noun- (chiefly attributive) A type of friction hitch.
- To climb a rope using a prusik.