Crossword Genius

With spy boss, unsuccessfully watch online channel (10)

Ross

I believe the answer is:

millstream

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'channel' is the definition.
(thesaurus)

'with spy boss unsuccessfully watch online' is the wordplay.
'with' means one lot of letters go next to another.
'spy boss unsuccessfully' becomes 'mill' (I can't justify this - if you can you should give a lot more credence to this answer).
'watch online' becomes 'stream' (I've seen this in another clue).
'mill' put next to 'stream' is 'MILLSTREAM'.

Can you help me to learn more?

(Other definitions for millstream that I've seen before include "Current power source" , "wheel turner?" , "Flow providing power to wheel" , "Race to power" .)

I've seen this clue in The Telegraph.
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