Driving force or impulse (7)
I believe the answer is:
impetus
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">driving force</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(impetus is a kind of driving force)</span><br/ ><br/ >'<span class="explanation-format__original">impulse</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for impetus that I've seen before include "Imputes to force with which a body moves" , "Incentive, stimulus" , "Motivation" , "influence" , "Drive" .)