Quarry’s last sign something’s not right after working stone (4)
I believe the answer is:
onyx
'stone' is the definition.
(type of gemstone)
'quarry's last sign something's not right after working' is the wordplay.
'last' indicates one should take the final letters.
'sign something's not right' becomes 'X' (a X can show something's wrong).
'after' says to put letters next to each other.
'working' becomes 'on' (on can mean working or activated).
The last letter of 'quarry' is 'y'.
'y'+'x'='yx'
'yx' after 'on' is 'ONYX'.
(Other definitions for onyx that I've seen before include "Agate with different layers of colour" , "Semi-precious variety of agate" , "Gemstone, a variety of chalcedony" , "Kind of agate" , "Variety of agate, used as a gemstone" .)