Work on a large stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'work on a large' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' means one lot of letters go next to another.
'large' becomes 'L' (eg in clothes sizes).
'op'+'a'+'l'='OPAL'
(Other definitions for opal that I've seen before include "Precious stone" , "Milky-white mineral" , "Milky stuff that's hard" , "Gem with delicate colours" , "what may be set in 29 [DIAL]" .)