You don’t even need the second <use>, you can just render the <circle> normally and use it in the clipPath:

   <svg width="200" height="200" viewBox="-100 -100 200 200">
     <defs>
       <clipPath id="ball">
  -      <circle cx="0" cy="20" r="70" />
  +      <use href="#round" />
       </clipPath>
     </defs>
  
  -  <circle cx="0" cy="20" r="70" fill="#D1495B" />
  +  <circle cx="0" cy="20" r="70" fill="#D1495B" id="round" />

(Personally I’d also drop the <defs>-wrapping. Never did understand why structuring them that way is recommended, I don’t think it actually gets you anything except for the vanishingly rare case where you want to put something that would render if it were not in a <defs> tag—I say “vanishingly rare” because situations like you showed—//defs/circle—should normally actually use <symbol>.)