Code word used in part of speech -- Oscar (4)

I believe the answer is:

echo

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'code word used in part of speech oscar' is the definition.
I know that 'oscar' relates to this answer.

'of speech oscar' is the wordplay.
'of' indicates a hidden word.
'ECHO' is hidden within 'speech oscar'.

Can you help me to learn more?

(Other definitions for echo that I've seen before include "Reminder" , "Evocation" , "Mimic" , "Reproduction" , "The distant sound" .)

I've seen this clue in The Telegraph.

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