Stone axes with no backs (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
'axes with no backs' is the wordplay.
'axes' becomes 'yx' (Y axis and X axis).
'with' means one lot of letters go next to another.
'backs' is a reversal indicator.
'no' backwards is 'on'.
'yx' after 'on' is 'ONYX'.
(Other definitions for onyx that I've seen before include "material for making cameos" , "Stone like marble with layered colours" , "Form of chalcedony" , "African antelope" , "Agate with alternating bands of colour" .)