An entrance of 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' (type of entrance).
'a'+'gate'='AGATE'
'of' acts as a link.
(Other definitions for agate that I've seen before include "Stone with coloured bands" , "Precious stone" , "Quartz with bands of colour, used in jewellery" , "Banded ornamental stone" , "On the way" .)