Impelling force (8)
I believe the answer is:
momentum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">impelling force</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for momentum that I've seen before include "Impetus gained by a moving object" , "Drive" , "Driving force, impetus" , "Impetus from motion" , "Impelling force" .)