The most common and most likely answer to this crossword clue is the 3 plus 3 plus 1 plus 6 letter word NOT FOR A MOMENT
N
O
T
F
O
R
A
M
O
M
E
N
T
Buddy explains!
The straight part
We know that "going off the rails/Never" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case going off the rails/never = not for a moment.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.