Stone barrier on the west of Alcatraz (5)
I believe the answer is:
agate
'stone' is the definition.
(agate is a type of stone)
'barrier on the west of alcatraz' is the wordplay.
'barrier' becomes 'gate' (gate is a kind of barrier).
'on' says to put letters next to each other.
'the west of' indicates taking the first letters (most west - leftmost - letter).
The first letter of 'alcatraz' is 'a'.
'gate' after 'a' is 'AGATE'.
(Other definitions for agate that I've seen before include "On the way" , "Hard stone with concentric bands of colour" , "Type of quartz" , "marble" , "rock with bands" .)