Work on a large gem (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'work on a large' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'on' 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 "Semi-precious stone of b;uish white colour" , "Birthstone for October" , "what may be set in 29 [DIAL]" , "often semi-transparent" , "Semiprecious stone of white and bluish colour" .)