Principal and master hissed in assembly (12)
I believe the answer is:
headmistress
'principal' is the definition.
(headmistress is a kind of principal)
'master hissed in assembly' is the wordplay.
'in assembly' indicates an anagram (letters assembled differently).
'master'+'hissed'='masterhissed'
'masterhissed' is an anagram of 'HEADMISTRESS'.
'and' acts as a link.
(Other definitions for headmistress that I've seen before include "Person in charge of a school" , "Female school leader" , "School principal" , "Teacher" .)