Brain tissue (4,6)
I believe the answer is:
grey matter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">brain tissue</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 grey matter that I've seen before include "Issue lacking clear boundaries" , "Intelligence; the brain" , "Brains" , "Brain tissue" , "intellectual capacity?" .)