Cram; matter (5)
I believe the answer is:
stuff
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">cram</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know this)</span><br/ ><br/ >'<span class="explanation-format__original">matter</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(stuff is a kind of matter)</span><br/ ><br/ ></span>
(Other definitions for stuff that I've seen before include "facts" , "12 [GLUT]" , "Miscellaneous objects" , "Matter" , "Material; eat lots" .)