An entrance with small stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'an entrance with small' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'with' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'gate'+'s'='AGATES'
(Other definitions for agates that I've seen before include "Ornamental stones" , "Hard stones with concentric bands of colour" , "Multi-coloured quartzes" , "Set aga up for shiny stones" , "Banded semiprecious stones" .)