The most common and most likely answer to this crossword clue is the 13 letter word TURTLE-NECKED
T
U
R
T
L
E
-
N
E
C
K
E
D
Buddy explains!
The straight part
We know that "reaching top of crag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reaching top of crag = turtle-necked.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.