Metal prong shortened (3)
I believe the answer is:
tin
'metal' is the definition.
(I know that tin is a type of metal)
'prong shortened' is the wordplay.
'prong' becomes 'tine' (tine is a kind of prong).
'shortened' means to remove the last letter.
'tine' with its last letter taken off is 'TIN'.
(Other definitions for tin that I've seen before include "A silvery-white metal, symbol Sn" , "Element, symbol Sn" , "Atomic number 50" , "Money" , "A silvery-white metal often in cans" .)