Principal element (4)
I believe this clue is a double definition.
'principal' is the first definition.
(synonyms)
'element' is the second definition.
(lead is a kind of element)
(Other definitions for lead that I've seen before include "Guide, conduct" , "initiative" , "first name in lights?" , "Toxic, malleable, metallic element" , "Live" .)
(Other definitions for staple that I've seen before include "The food one normally eats" , "Paper pin - basic" , "Basic element" , "It holds fast" , "e.g. rice, sweetcorn ..." .)