Stone circle, mostly white (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'circle mostly white' is the wordplay.
'circle' becomes 'O' (O is a circle).
'mostly' means to remove the last letter (most but not all of the word).
'white' becomes 'pale' (I've seen this before).
'pale' with its last letter taken off is 'pal'.
'o'+'pal'='OPAL'
(Other definitions for opal that I've seen before include "raw material" , "what may be set in 29 [DIAL]" , "Shimmering stone" , "Delicately coloured gem" , "Milky-white gemstone with flashes of colour" .)