White mineral artist found in chest (5)
I believe the answer is:
borax
'white mineral' is the definition.
(borax is a white mineral)
'artist found in chest' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'found in' means one lot of letters goes inside another.
'chest' becomes 'box' (both can mean a square container).
'ra' going into 'box' is 'BORAX'.
(Other definitions for borax that I've seen before include "White salt" , "Hydrated mineral" .)