Case of spy using communication device for tapping (9)

I believe the answer is:

syphoning

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'communication device for tapping' is the definition.
I can't judge whether this definition defines the answer.

'case of spy using' is the wordplay.
'case of' means to remove the middle letters (outsides of).
'using' becomes 'phoning' (I can't explain this - if you can you should believe this answer much more).
'spy' with its centre taken out is 'sy'.
'sy'+'phoning'='SYPHONING'

Can you help me to learn more?

I've seen this clue in The Independent.

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