Immature and sneaky pest (8)

I believe the answer is:

greenfly

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'pest' is the definition.
(I've seen this before)

'immature and sneaky' is the wordplay.
'immature' becomes 'green' (synonyms).
'and' says to put letters next to each other.
'sneaky' becomes 'fly' (I can't explain this - if you can you should give a lot more credence to this answer).
'green'+'fly'='GREENFLY'

Can you help me to learn more?

(Other definitions for greenfly that I've seen before include "Common aphids" , "Common aphid" , "Pests" , "Garden pest" .)

I've seen this clue in The Times.

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