Rectangular block of cut stone (6)
I believe the answer is:
ashlar
'rectangular block of cut stone' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ashlar that I've seen before include "Masonry made of large square-cut stones" , "building material" , "Dressed stone for facing (A, not E)" , "Rectangular block of hewn stone" , "Dressed stone block" .)