An entrance in store (5)
I believe the answer is:
agate
'store' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'an entrance' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'a'+'gate'='AGATE'
'in' is the link.
Can you help me to learn more?
(Other definitions for agate that I've seen before include "Type of quartz" , "Old drama critic" , "Striped stone" , "marble" , "On the way" .)