Plug broken by Greek character, little pest (5)
I believe the answer is:
aphid
'little pest' is the definition.
(insect considered a pest)
'plug broken by greek character' is the wordplay.
'plug' becomes 'ad' (as in plugging a product).
'broken by' is an insertion indicator (inserted letters break into the word).
'greek character' becomes 'phi' (letter from the Greek alphabet).
'ad' going around 'phi' is 'APHID'.
(Other definitions for aphid that I've seen before include "plant pest" , "Blackfly or greenfly" , "Sap-sucking bug" , "Plant-louse" , "Horticultural pest" .)