An entrance made of stone (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'an entrance' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'a'+'gate'='AGATE'
'made of' acts as a link.
(Other definitions for agate that I've seen before include "marble" , "Banded ornamental stone" , "On the way" , "Variety of chalcedony" , "Impure form of quartz with banded chalcedony, used as a gemstone" .)