Work on a large gemstone (4)
I believe the answer is:
opal
'gemstone' is the definition.
(opal is a kind of gemstone)
'work on a large' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'large' becomes 'L' (eg in clothes sizes).
'op'+'a'+'l'='OPAL'
(Other definitions for opal that I've seen before include "Milky-white gemstone with flashes of colour" , "Variegated gemstone" , "Milky-white gem" , "One's stony" , "Mineral, usually iridescent" .)