Metal used by Mercedes-Benz in cars (4)
I believe the answer is:
zinc
'metal' is the definition.
(zinc is a metal)
'used by mercedes-benz in cars' is the wordplay.
'used by' indicates a hidden word.
'ZINC' can be found hidden inside 'mercedes benz in cars'.
(Other definitions for zinc that I've seen before include "Metal used for galvanising steel" , "White metallic element used in alloys" , "Blusish-white lustrous metallic element" , "Metallic element present with copper in brass" , "It is used to coat iron against corrosion" .)