Principal – advantage (4)
I believe the answer is:
lead
This clue is a double definition.
'principal' is the first definition.
(I know that principal can be written as lead)
'advantage' is the second definition.
(I know that lead is a type of advantage)
(Other definitions for lead that I've seen before include "Main news story" , ""bullets"" , "Accompany to show the way" , "Short wire used as jumper - Pb" , "Be in charge of; metal" .)