Crossword Genius

An entrance in store (5)

Ross

I believe the answer is:

agate

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'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" .)

I've seen this clue in the Evening Standard.
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