Plan to put layers on something valuable (9)
I believe the answer is:
stratagem
'plan' is the definition.
(stratagem is a kind of plan)
'layers on something valuable' is the wordplay.
'layers' becomes 'strata' (stratum is a kind of layer).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'something valuable' becomes 'gem'.
'strata'+'gem'='STRATAGEM'
'to put' acts as a link.
(Other definitions for stratagem that I've seen before include "Start game with cunning plan" , "Tag master with cunning plan" , "could be a trick" , "Cunning manoeuvre" , "Cunning plot" .)