Stone in large masses (3)
I believe the answer is:
gem
'stone' is the definition.
(thesaurus)
'in large masses' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GEM' can be found hidden inside 'large masses'.
(Other definitions for gem that I've seen before include "Valued person; jewel" , "Half of 7 [SIGN OF THE ZODIAC]" , "Emerald or sapphire, say" , "one of the Cairngorms?" , "Treasured person" .)