Locomotive power (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">locomotive power</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 steam that I've seen before include "Vapourised water" , "Invisible gas from boiling water" , "Travel in liner" , "Vaporous exhalation" , "Sort of water" .)