Work with a learner to produce a gem! (4)
I believe the answer is:
opal
'a gem' is the definition.
(opal is a kind of gem)
'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'
'to produce' is the link.
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "Form of hydrated silica, often used as a gemstone" , "Many-coloured gem" , "Jewellery item" , "little jewel" .)