The most common and most likely answer to this crossword clue is the 9 letter word KEYHOLDER
I've seen this crossword clue in The Times
K
E
Y
H
O
L
D
E
R
Buddy explains!
The straight part
We know that "a trusted opener" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a trusted opener = keyholder.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 KEYHOLDER mean?
KEYHOLDER noun- One who holds the key to a building etc. for security reasons.
- The individual or organisation with access to a cryptographic key.