Brain cell (6)
I believe the answer is:
neuron
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">brain cell</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for neuron that I've seen before include "Nerve impulse cell" , "Transmitting nerve cell" , "It transmits nerve impulses" , "Impulsive thing" , "Cell that conducts nerve impulses" .)