Crossword Genius

Work with a learner on a stone (4)

Ross

I believe the answer is:

'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'.
'op'+'a'+'l'='OPAL'

'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".)