Jailed German boy, cunning little pest (7,3)

I believe the answer is:

hessian fly

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'little pest' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'jailed german boy cunning' is the wordplay.
'jailed german boy' becomes 'hessian' (I can't justify this - if you can you should believe this answer much more).
'cunning' becomes 'fly' (fly can mean skilful or sharp).
'hessian'+'fly'='HESSIAN FLY'

Can you help me to learn more?

I've seen this clue in The Guardian.

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