Lacking experience with cunning garden pests (8)
I believe the answer is:
'garden pests' is the definition.
'greenfly' can be an answer for 'pests' (I've seen this before). I am not certain of the 'garden' bit.
'lacking experience with cunning' is the wordplay.
'lacking experience with' becomes 'green' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cunning' becomes 'fly' (fly can mean skilful or sharp).
Can you help me to learn more?
(Other definitions for greenfly that I've seen before include "Pests" , "Garden pest" , "Common aphids" , "Common aphid" .)