Large trunk artery (5)
I believe the answer is:
aorta
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">large trunk artery</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for aorta that I've seen before include "This carries blood from the heart" , "Major blood vessel" , "Main artery of body" , "Main artery from heart" , "sort of vessel" .)