Narrow opening or fissure (7)
I believe the answer is:
crevice
This clue is a double definition.
'narrow opening' is the first definition.
(I've seen this in another clue)
'fissure' is the second definition.
(I know that fissure can be written as crevice)
(Other definitions for crevice that I've seen before include "Cleft, fissure" , "Split" , "Crack or fissure." , "Narrow crack" , "Narrow rock fissure" .)