Work with a learner on a stone (4)
'stone' is the definition.
(I've seen this before)
'work with a learner' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'with' says to put letters next to each other.
'learner' becomes 'l'.
'on a' acts as a link.
(Other definitions for opal that I've seen before include "One in a ring perhaps", "Milky white gem", "Jewellery item", "Mineral used as a gemstone, of hydrated silica", "One's stony".)