Working cemetery's last cross in stone (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
'working cemetery's last cross' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'last' says to take the final letters.
'cross' becomes 'x' (X is cross-shaped).
The last letter of 'cemetery' is 'y'.
'on'+'y'+'x'='ONYX'
'in' is the link.
(Other definitions for onyx that I've seen before include "Striped agate" , "Stone-like marble with different colours in layers" , "Variety of agate used for making cameos" , "Variety of agate, used as a gemstone" , "Smooth stone with bands of colour" .)