Plan getting precious stone under layers of rock (9)
I believe the answer is:
stratagem
'plan' is the definition.
(stratagem is a kind of plan)
'precious stone under layers of rock' is the wordplay.
'precious stone' becomes 'gem' ('gem' can be a synonym of 'precious stone').
'under' says to put letters next to each other.
'layers of rock' becomes 'strata' (I've seen this in other clues).
'gem' put after 'strata' is 'STRATAGEM'.
'getting' is the link.
(Other definitions for stratagem that I've seen before include "Devious ploy" , "way to outwit opponent" , "Plan to deceive enemy" , "Crafty manoeuvre" , "could be a trick" .)