This is a double definition - the clue gives you two different meanings that both lead to the same answer word.
Look for a word that means 'copy' but also represents the letter 'E' in a well-known spelling alphabet used by military and aviation.
Answer: ECHO | This is a double definition: ECHO represents the letter 'E' in the NATO phonetic alphabet, and an echo is also a 'copy' (like a sound that repeats or copies the original).