An entrance 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'
'of' acts as a link.
(Other definitions for agate that I've seen before include "marble" , "Decorative stone" , "Old drama critic" , "Variety of chalcedony" , "Stone with coloured bands" .)