Open with a letter opener (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
slit
'open with a letter opener' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for slit that I've seen before include "Narrow rent" , "Narrow aperture" , "Long narrow cut" , "Narrow straight cut or opening" , "Long narrow opening" .)