Thrust, impetus (7)
I believe the answer is:
impulse
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">thrust</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(impulse is a kind of thrust)</span><br/ ><br/ >'<span class="explanation-format__original">impetus</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that impetus can be written as impulse)</span><br/ ><br/ ></span>
(Other definitions for impulse that I've seen before include "Chew" , "Miles up (anag)" , "Whim; thrust" , "A sudden inclination to act (7)" , "Instinctive motive" .)