Sap-sucking insect (5)
I believe the answer is:
aphid
'sap-sucking insect' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aphid that I've seen before include "plant pest" , "Small plant-sucking insect" , "Insect feeding on plant juices" , "Plant louse" , "one living on juices" .)