Railway locomotive (6)
I believe the answer is:
engine
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">railway locomotive</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for engine that I've seen before include "Machine supplying power" , "Machine doing work" , "Machine converting energy into motion" , "machinery" , "Power unit" .)