On reflection, instructed a jeweller to incorporate this stone (4)
I believe the answer is:
jade
'this stone' is the definition.
(I've seen this before)
'on reflection instructed a jeweller to incorporate' is the wordplay.
'on reflection' says the letters should be written backwards.
'to incorporate' says the answer is hidden in the clue (I've seen 'incorporates' mean this).
'edaj' is hidden in the letters of 'instructed a jeweller'.
'edaj' backwards is 'JADE'.
(Other definitions for jade that I've seen before include "Overworked horse; stone" , "Semi-precious gemstone -- tired old horse" , "Greenish, semi-precious stone" , "Green stone; hack" , "Worthless horse" .)