Phone assistant knocks us back endlessly (4)

I believe the answer is:

siri

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'phone assistant' is the definition.
(iPhone voice assistant)

'knocks us back endlessly' is the wordplay.
'knocks us' becomes 'sir' (I can't justify this - if you can you should believe this answer much more).
'back' becomes 'aid' (I've seen this in another clue).
'endlessly' indicates the central letters (the ends of the words are removed).
The central letter of 'aid' is 'i'.
'sir'+'i'='SIRI'

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(Another definition for siri that I've seen is " Apple's virtual assistant".)

I've seen this clue in The Irish Times.

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