Work by a pupil is a gem (4)
I believe the answer is:
opal
'a gem' is the definition.
(opal is a kind of gem)
'work by a pupil' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'by' means one lot of letters go next to another.
'pupil' becomes 'l'.
'op'+'a'+'l'='OPAL'
'is' is the link.
(Other definitions for opal that I've seen before include "raw material" , "Milk-coloured gemstone" , "Semi-precious stone of b;uish white colour" , "Many-coloured gem" , "Gem-stone with a milky quality" .)