Gem obtained for a pound after work (4)
I believe the answer is:
opal
'gem' is the definition.
(I know that opal is a type of opaque gem)
'a pound after work' is the wordplay.
'pound' becomes 'L' (the pound sign is based on an L).
'after' is a charade indicator (letters next to each other).
'work' becomes 'op' (abbreviation for opus).
'a'+'l'='al'
'al' put after 'op' is 'OPAL'.
'obtained for' acts as a link.
(Other definitions for opal that I've seen before include "Quartz-like gemstone" , "Form of hydrated silica, often used as a gemstone" , "little jewel" , "Birthstone for October" , "Translucent gem" .)