The most common and most likely answer to this crossword clue is the 8 letter word ADHERING
I've seen this crossword clue in The Times
A
D
H
E
R
I
N
G
Buddy explains!

The straight part
We know that "holding fast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case holding fast = adhering.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 ADHERING mean?
ADHERING verb- To stick fast or cleave, as a glutinous substance does; to become joined or united.
- To be attached or devoted by personal union, in belief, on principle, etc.
- To be consistent or coherent; to be in accordance; to agree.
- To affirm a judgment.