Organic matter in soil (5)
I believe the answer is:
humus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">organic matter in soil</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 humus that I've seen before include "Decomposed vegetation" , "Rich organic material from dead leaves, plants etc" , "Organic matter in topsoil" , "Compost" , "Organic stuff" .)