A metal? A metal in fruit! (8)
I believe the answer is:
platinum
'a metal?' is the definition.
(I know that platinum is a metal)
'a metal in fruit' is the wordplay.
'metal' becomes 'tin' (tin is a kind of metal).
'in' means one lot of letters goes inside another.
'fruit' becomes 'plum' (plum is a kind of fruit).
'a'+'tin'='atin'
'atin' put inside 'plum' is 'PLATINUM'.
(Other definitions for platinum that I've seen before include "Valuable stuff, after a grind" , "Valuable silvery metal" , "Jewellers' metal" , "12 down metal" , "Metal used in jewellery" .)