Important, seemingly heavy role (4)
I believe the answer is:
lead
'role' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for lead that I've seen before include "Metal - wire" , "Leash -- Pb" , "Command; metal" , "Be ahead of - guide" , "Guide; metallic element" .)