Principal nut (4)
I believe the answer is:
head
This clue is a double definition.
'principal' is the first definition.
(synonyms)
'nut' is the second definition.
(I've seen this in other clues)
(Other definitions for head that I've seen before include "Leader; water pressure" , "Beer foam; front" , "River or stream source" , "John on board" , "Leader; front" .)