An opening in stone (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'an opening' is the wordplay.
'an' becomes 'a'.
'opening' becomes 'gate' (a gate is an opening or entrance).
'a'+'gate'='AGATE'
'in' is the link.
(Other definitions for agate that I've seen before include "On the way" , "Hard stone with concentric bands of colour" , "Ornamental form of quartz" , "Variety of chalcedony" , "marble" .)