The most common and most likely answer to this crossword clue is the 8 letter word LATCHKEY
I've seen this crossword clue in The Daily Telegraph
L
A
T
C
H
K
E
Y
More clues leading to the result LATCHKEY
Buddy explains!

The straight part
We know that "opener" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opener = latchkey.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 LATCHKEY mean?
LATCHKEY noun- A key, especially to an outside door.
- A child who is given a key to the home and is expected to remain at home alone (without adult supervision until the parents return from work).
- Equipped with a key; generally in the phrase latchkey child.