The most common and most likely answer to this crossword clue is the 6 plus 3 letter word STAPLE GUN
I've seen this crossword clue in The Sunday Telegraph
S
T
A
P
L
E
G
U
N
More clues leading to the result STAPLE GUN
Buddy explains!
The straight part
We know that "Fastener" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fastener = staple gun.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.