Small plant-sucking insects (6)
I believe the answer is:
aphids
'small plant-sucking insects' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for aphids that I've seen before include "Small insects like greenfly or blackfly" , "Plant pests" , "Plant-lice like greenflies" , "Plant lice like greenfly" , "Garden pests" .)