An entrance in ornamental stone (5)
I believe the answer is:
agate
'ornamental stone' is the definition.
(I've seen this before)
'an entrance' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (I've seen this before**).
'a'+'gate'='AGATE'
'in' is the link.
(Other definitions for agate that I've seen before include "Onyx" , "Precious stone" , "Decorative stone" , "Banded ornamental stone" , "Impure form of quartz with banded chalcedony, used as a gemstone" .)