Ploys to find precious stones behind layers of rock (10)
I believe the answer is:
stratagems
'ploys' is the definition.
('stratagem' can be a synonym of 'ploy')
'precious stones behind layers of rock' is the wordplay.
'precious stones' becomes 'gems' ('gem' can be a synonym of 'precious stone').
'behind' says to put letters next to each other.
'layers of rock' becomes 'strata' (I've seen this before).
'gems' put after 'strata' is 'STRATAGEMS'.
'to find' is the link.
(Other definitions for stratagems that I've seen before include "Plans" , "Manoeuvres" .)