Plant louse or greenfly (5)
I believe the answer is:
aphid
This clue is a double definition.
'plant louse' is the first definition.
(aphid is a kind of plant louse)
'greenfly' is the second definition.
(I've seen this before)
(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" .)