Principal role — heavy metal (4)
I believe the answer is:
lead
This clue is a double definition.
'principal role' is the first definition.
(I've seen this in another clue)
'heavy metal' is the second definition.
(lead is a heavy metal)
(Other definitions for lead that I've seen before include "Travel in front of" , "Take a position in front" , "Main news story" , "Metal -- Pb" , "Guide by going in front" .)