Pesto ingredient (5)
I believe the answer is:
basil
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pesto ingredient</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 basil that I've seen before include "serially chaotic character" , "A herb; Cardinal --- Hume" , "Herb; man's name" , "Mediterranean herb" , "Boy - herb" .)