The most common and most likely answer to this crossword clue is the 7 letter word RECLIMB
I've seen this crossword clue in Independent
R
E
C
L
I
M
B
Buddy explains!
The straight part
We know that "Not for the first time, go up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case not for the first time, go up = reclimb.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.