One dismissed from principal staff (3)
I believe the answer is:
man
'staff' is the definition.
(manning is a kind of staffing)
'one dismissed from principal' is the wordplay.
'one' becomes 'i' (Roman numeral).
'dismissed from' indicates named letters should be taken away.
'principal' becomes 'main' (I've seen this before).
'main' with 'i' taken away is 'MAN'.
(Other definitions for man that I've seen before include "Isle of -; chess piece" , "Chap" , "race" , "Staff the island" , "Geezer" .)