Stone used in constructing an entrance (5)
I believe the answer is:
agate
'stone' is the definition.
(I know that agate is a type of chalcedony)
'used in constructing an entrance' is the wordplay.
'used in' indicates the central letters (the letters used on the inside of the word).
'constructing' becomes 'retracing' ('retrace' can be a synonym of 'construct').
'an entrance' becomes 'gate' (type of entrance).
The central letter of 'retracing' is 'a'.
'a'+'gate'='AGATE'
(Other definitions for agate that I've seen before include "Impure form of quartz with banded chalcedony, used as a gemstone" , "Decorative stone" , "Stone with coloured bands" , "Variety of chalcedony" , "Old drama critic" .)