Reminder of the code word for an E (4)
I believe the answer is:
echo
I believe this clue is a double definition.
'reminder' is the first definition.
(an echo is a reminder of something)
'the code word for an e' is the second definition.
(E for Echo in the NATO phonetic alphabet)
'of' acts as a link.
(Other definitions for echo that I've seen before include "'Greek nymph, she pined away until only her voice remained (4)'" , "Resound; (radar) return" , "Languishing nymph" , "Nymph (only able to repeat the last words of others)" , "ring" .)