Plant louse or greenfly (5)
I believe the answer is:
aphid
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">plant louse</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(aphid is a kind of plant louse)</span><br/ ><br/ >'<span class="explanation-format__original">greenfly</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for aphid that I've seen before include "Blackfly or greenfly" , "Small insect like greenfly" , "Insect feeding on plant juices" , "Sap-sucking bug" , "Fly" .)