Gem of work taking a line (4)
I believe the answer is:
opal
'gem' is the definition.
(opal is a kind of gem)
'work taking a line' is the wordplay.
'work' becomes 'op' (abbreviation for opus**).
'taking' says to put letters next to each other.
'line' becomes 'l'.
'op'+'a'+'l'='OPAL'
'of' is the link.
(Other definitions for opal that I've seen before include "Lustrous stone" , "Milky stuff that's hard" , "Delicately-coloured gem" , "Semi-transparent gem" , "Opaque gem" .)