Principal metal (4)
I believe the answer is:
lead
I believe this clue is a double definition.
'principal' is the first definition.
(I know that principal can be written as lead)
'metal' is the second definition.
(I know that lead is a type of metal)
(Other definitions for lead that I've seen before include "First put down spade, say" , "Primary" , "writing material" , "Precede; element" , "Guide, direct" .)