Island in plaza renovated with evenly built-in stone (5,6)
I believe the answer is:
Here is my best explanation:
'stone' is the definition.
(I've seen this before)
'island in plaza renovated with evenly built-in' is the wordplay.
'island' becomes 'isl' (I can't justify this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'renovated' is an anagram indicator.
'with' says to put letters next to each other.
'evenly' means one should take alternating letters (even-numbered letters only).
The alternate letters of 'builtin' are 'uli'.
'plaza' with letters rearranged gives 'lapaz'.
'isl' going within 'lapaz' is 'lapislaz'.