The most common and most likely answer to this crossword clue is the 9 letter word PADLOCKED
I've seen this crossword clue in The Sunday Telegraph
P
A
D
L
O
C
K
E
D
Buddy explains!
The straight part
We know that "securely fastened" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case securely fastened = padlocked.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 PADLOCKED mean?
PADLOCKED verb- To lock using a padlock.