Type of school principal (7)
I believe the answer is:
primary
I believe this is a double definition.
'type of school' is the first definition.
'principal' is the second definition.
(I know that principal can be written as primary)
(Other definitions for primary that I've seen before include "The colours from which all others can be made" , "Paramount" , "Principal" , "Of the first order" , "Main" .)