Answer letter from Nato? (4)

I believe the answer is:

echo

I believe this is a double definition.

'answer' is the first definition.
(thesaurus)

'letter from nato?' is the second definition.
(NATO phonetic alphabet)

(Other definitions for echo that I've seen before include "letter we hear" , "Nymph whose love Narcissus spurned" , "Bounced sound" , "Resound; (radar) return" , "Repeating of sound" .)

I've seen this clue in The Telegraph.

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