Work a large stone (4)
I believe the answer is:
opal
'stone' is the definition.
(thesaurus)
'work a large' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'large' becomes 'l'.
'op'+'a'+'l'='OPAL'
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "Semi-precious stone of b;uish white colour" , "Variegated gemstone" , "Milky-white gemstone with flashes of colour" , "Something milky-white" .)