Plant louse (5)
I believe the answer is:
aphid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plant louse</span>' is the definition.<br/ ><span class="explanation-format__explanation">(aphid is a kind of plant louse)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aphid that I've seen before include "Sap-sucking bug" , "Greenfly etc" , "Plant-louse" , "plant pest" , "Small plant-sucking insect" .)