Sudden whim (7)
I believe the answer is:
impulse
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sudden whim</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 impulse that I've seen before include "Whim; thrust" , "Miles up (anag)" , "A sudden urge or whim" , "I may cause desire" , "Sudden urge to do something" .)