Horse’s fastest pace (6)
I believe the answer is:
gallop
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">horse's fastest pace</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for gallop that I've seen before include "Horse's fastest pace" , "A fast canter" , "'Run at a high speed, like a horse (6)'" , "Fast gait of a horse" , "Fastest run of a horse" .)