Metal left inside idol (4)
I believe the answer is:
gold
'metal' is the definition.
(I know that gold is a precious metal)
'left inside idol' is the wordplay.
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'idol' becomes 'god' (synonyms).
'l' going within 'god' is 'GOLD'.
(Other definitions for gold that I've seen before include "Mine produces this" , "Wealth; first prize" , "Valuable metal" , "Deep yellow" , "Woman after a man's money" .)