Stone top of arched entrance (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'top of arched entrance' is the wordplay.
'top of' says to take the initial letters.
'entrance' becomes 'gate' (type of entrance).
The first letter of 'arched' is 'a'.
'a'+'gate'='AGATE'
(Other definitions for agate that I've seen before include "Banded ornamental stone" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Type of quartz" , "rock with bands" , "Quartz with bands of colour, used in jewellery" .)