Principals in school ignored racist title (3)
I believe the answer is:
sir
'title' is the definition.
(respectful address for a man)
'principals in school ignored racist' is the wordplay.
'principals in' suggests taking the first letters.
The initial letters of 'school ignored racist' is 'SIR'.
(Other definitions for sir that I've seen before include "Title given to 10 Across" , "Baronet's title" , "Term of address for a gentleman" , "Honorary male title" , "Title of 1 across" .)