Grey matter (5)
I believe the answer is:
brain
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">grey matter</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 brain that I've seen before include "Seat of the intellect" , "Intellectual capacity" , "Hit hard; intellect" , "and 15 ac. The loss of experts through emigration" , "Encephalon" .)