Be first with heavy metal (4)
I believe the answer is:
lead
This clue is a quadruple definition!!
'be' is the first definition.
(leading is a kind of being)
'first' is the second definition.
(lead can mean leading or primary)
'with' acts as a link.
'heavy' is the third definition.
(thesaurus)
'metal' is the fourth definition.
(I know that lead is a type of metal)
(Other definitions for lead that I've seen before include "Accompany to show the way" , "first guitarist" , "spearhead" , "Command; metal" , "Metal -- Pb" .)