The most common and most likely answer to this crossword clue is the 8 letter word SECURELY
I've seen this crossword clue in The Guardian
S
E
C
U
R
E
L
Y
Buddy explains!

The straight part
We know that "Fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fast = securely.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 SECURELY mean?
SECURELY adverb- (manner) In a secure manner; without fear or apprehension; without danger