An entrance has small stones (6)
I believe the answer is:
agates
'stones' is the definition.
(I've seen this before)
'an entrance has small' is the wordplay.
'an' becomes 'a'.
'entrance' becomes 'gate' (type of entrance).
'has' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'gate'+'s'='AGATES'
(Other definitions for agates that I've seen before include "Multi-coloured quartzes" , "Set aga up for shiny stones" , "Hard stones with concentric bands of colour" , "Hard coloured stones from a stage" , "Stones to be found in a stage" .)