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' means one lot of letters go next to another.
'layers of rock' becomes 'strata' (I've seen this in another clue).
'gems' put after 'strata' is 'STRATAGEMS'.
'to find' acts as a link.
(Other definitions for stratagems that I've seen before include "Plans" , "Manoeuvres" .)