Stone-working axes (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
'working axes' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'axes' becomes 'yx' (Y axis and X axis).
'on'+'yx'='ONYX'
(Other definitions for onyx that I've seen before include "Form of agate" , "Gemstone, a variety of chalcedony" , "Black and white striped gemstone" , "Banded gemstone" , "Sort of agate" .)