Knight or teacher (3)
I believe the answer is:
sir
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">knight</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(a knight has the title Sir)</span><br/ ><br/ >'<span class="explanation-format__original">teacher</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(way of addressing a male teacher)</span><br/ ><br/ ></span>
(Other definitions for sir that I've seen before include "as Winston Churchill was" , "Form of address for a knight" , "Noble title of undesirable kind" , "Baronet's title" , "Title for knight or teacher" .)