It's a stone circle, mate (4)
I believe the answer is:
opal
'it's a stone' is the definition.
(I've seen this before)
'circle mate' is the wordplay.
'circle' becomes 'o'.
'mate' becomes 'pal' (I've seen this before).
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "Translucent gemstone" , "Semi-precious stone of milky or bluish colour" , "Milky-white gemstone with flashes of colour" , "Birthstone for October" , "Gem stone" .)