Indigo plant (4)
I believe the answer is:
anil
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">indigo plant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(anil is a kind of indigo plant)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for anil that I've seen before include "Colouring" , "It produces dye" , "Shrub yielding indigo" , "name of man" , "Species of blue" .)