A narrow opening, stone (5)
I believe the answer is:
agate
'stone' is the definition.
(I've seen this before)
'a narrow opening' is the wordplay.
'narrow opening' becomes 'gate' (I've seen this before).
'a'+'gate'='AGATE'
(Other definitions for agate that I've seen before include "Impure form of quartz with banded chalcedony, used as a gemstone" , "Semi-precious quartz" , "Quartz gem" , "rock with bands" , "Onyx" .)