Plan to find valuable stone under layers of rock (9)
I believe the answer is:
stratagem
'plan' is the definition.
(stratagem is a kind of plan)
'valuable stone under layers of rock' is the wordplay.
'valuable stone' becomes 'gem' (I've seen this before).
'under' means one lot of letters go next to another.
'layers of rock' becomes 'strata' (I've seen this in other clues).
'gem' put after 'strata' is 'STRATAGEM'.
'to find' acts as a link.
(Other definitions for stratagem that I've seen before include "Clever scheme" , "Ploy - start game (anag)" , "Tag master with cunning plan" , "Manoeuvre - Ma's target (anag)" , "A cunning plan or scheme" .)