A narrow opening for Rob from Bristol (4)
I believe the answer is:
slit
'a narrow opening' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for slit that I've seen before include "Narrow aperture" , "Long narrow cut" , "Long straight cut" , "Narrow rent" , "Narrow straight cut or opening" .)