The most common and most likely answer to this crossword clue is the 6 plus 3 letter word LETTER BOX
I've seen this crossword clue in The Guardian
L
E
T
T
E
R
B
O
X
More clues leading to the result LETTER BOX
- What security-conscious Spooner has fitted to front door
- What security-concious Spooner has fitted to front door
- Post office facility gets improved security from Spooner
- Slot for delivering Spooner’s advice for a safer home
- Where to post Cockney’s plea for female pugilism?
- Door opening, Spooner discussing enhanced security system?
- Blank space for you, dear solver!
Buddy explains!
The straight part
We know that "Door opening" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case door opening = letter box.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.