Work with a learner on a stone (4)
I believe the answer is:
opal
'stone' is the definition.
(gemstone)
'work with a learner' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'with' means one lot of letters go next to another.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'op'+'a'+'l'='OPAL'
'on a' is the link.
(Other definitions for opal that I've seen before include "Milky stuff that's hard" , "Iridescent quartz-like stone" , "item in Hatton Garden?" , "Semi-precious stone of b;uish white colour" , "Form of hydrated silica, often used as a gemstone" .)