Natural fertiliser (6)
I believe the answer is:
manure
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">natural fertiliser</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 manure that I've seen before include "Organic fertiliser" , "Dung as fertiliser" , "Crap" , "Muck" , "Animal dung used as fertilizer" .)