Head of a train (6)
I believe the answer is:
engine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">head of a train</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for engine that I've seen before include "Machine supplying power" , "Machine with moving parts" , "Motive machine" , "Power unit" , "Locomotive" .)