Work with a large gem (4)
I believe the answer is:
Here is my best explanation:
'gem' is the definition.
(opal is a kind of gem)
'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'.
(Other definitions for opal that I've seen before include "Precious stone", "Stone", "Colour", "Iridescent quartz-like stone", "Birthstone for October", "Milky white gem", "Gemstone mineral", "Jewel", "Gemstone, hydrated silica" and "Opaque mineral used a gemstone".)