Manoeuvre to find precious stone under geological layers (9)
I believe the answer is:
stratagem
'manoeuvre' is the definition.
(stratagem is a kind of manoeuvre)
'precious stone under geological layers' is the wordplay.
'precious stone' becomes 'gem' ('gem' can be a synonym of 'precious stone').
'under' says to put letters next to each other.
'geological layers' becomes 'strata' (I've seen this before).
'gem' after 'strata' is 'STRATAGEM'.
'to find' is the link.
(Other definitions for stratagem that I've seen before include "Ploy - start game (anag)" , "way to outwit opponent" , "Devious ploy" , "Plan to deceive enemy" , "Start game with cunning plan" .)