Process of slowing down (12)
I believe the answer is:
deceleration
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">process of slowing down</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 deceleration that I've seen before include "Process of slowing down" , "Slowing of velocity" .)