Principal teachers (5)
I believe the answer is:
heads
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">principal teachers</span>' is the definition.<br/ ><span class="explanation-format__explanation">(headteachers)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for heads that I've seen before include "Ship's latrine; addicts" , "Leaders" , "Noddles; nuts" , "School principals" , "a 50-50 chance" .)