Stone barrier adjoining rear of villa (5)
I believe the answer is:
agate
'stone' is the definition.
(I know that agate is a type of chalcedony)
'barrier adjoining rear of villa' is the wordplay.
'barrier' becomes 'gate' (gate is a kind of barrier).
'adjoining' means one lot of letters go next to another.
'rear of' says to take the final letters.
The last letter of 'villa' is 'a'.
'gate' after 'a' is 'AGATE'.
(Other definitions for agate that I've seen before include "Quartz with bands of colour, used in jewellery" , "Impure form of quartz with banded chalcedony, used as a gemstone" , "Hard stone with concentric bands of colour" , "Semi-transparent stone" , "Semi-precious stone" .)