Work with a large stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'work with a large' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' says to put letters next to each other.
'large' becomes 'L' (eg in clothes sizes).
'op'+'a'+'l'='OPAL'
(Other definitions for opal that I've seen before include "Translucent gemstone" , "Variegated gemstone" , "Translucent gem" , "Iridescent quartz-like stone" , "it may be highly valued" .)