Plot contains treasure under layers of rock (9)
I believe the answer is:
stratagem
'plot' is the definition.
(thesaurus)
'treasure under layers of rock' is the wordplay.
'treasure' becomes 'gem' ('gem' can be a synonym of 'treasure').
'under' is a charade indicator (letters next to each other).
'layers of rock' becomes 'strata' (I've seen this in other clues).
'gem' after 'strata' is 'STRATAGEM'.
'contains' acts as a link.
(Other definitions for stratagem that I've seen before include "Plan to deceive enemy" , "Cunning plot" , "Ruse" , "Manoeuvre - Ma's target (anag)" , "Clever scheme" .)