Mailbox opening (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
slot
'mailbox opening' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slot that I've seen before include "Lots (anag.)" , "Narrow opening as in gaming machine" , "Hole for e.g. coin" , "Long narrow aperture" , "Long narrow slit" .)