An entrance in stone (5)
I believe the answer is:
agate
'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 "Hard stone with concentric bands of colour" , "Precious stone" , "Quartz variety" , "Decorative stone" , "Variegated chalcedony" .)