Plant louse; garden pest (5)
I believe the answer is:
aphid
I believe this is a double definition.
'plant louse' is the first definition.
(aphid is a kind of plant louse)
'garden pest' is the second definition.
(I've seen this before)
(Other definitions for aphid that I've seen before include "little sucker" , "Garden pest" , "Plant-louse" , "one living on juices" , "Greenfly etc" .)