Jockey replaces these for a Grand National runner, say (13)

I believe the answer is:
steeplechaser
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a grand national runner say</span>' is the definition.<br/ ><span class="explanation-format__explanation">(the Grand National is a steeplechase)</span><br/ ><br/ >'<span class="explanation-format__original">jockey replaces these</span>' is the wordplay.<br/ >'<span class="explanation-format__original">jockey</span>' is an anagram indicator <span class="explanation-format__explanation">(the letters scramble or jockey around)</span>.<br/ >'<span class="explanation-format__original">replaces</span>'+'<span class="explanation-format__original">these</span>'='<span class="explanation-format__intermediate">replacesthese</span>'<br/ >'<span class="explanation-format__intermediate">replacesthese</span>' anagrammed gives '<span class="explanation-format__answer">STEEPLECHASER</span>'.<br/ ><br/ >'<span class="explanation-format__original">for</span>' acts as a link.<br/ ></span>
(Other definitions for steeplechaser that I've seen before include "Competitive runner" , "Racehorse - athlete" , "Cross-country runner" , "Racehorse; 13" , "Horse in Grand National, for example" .)