Delta neighbor
I believe the answer is:
epsilon
'delta neighbor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for epsilon that I've seen before include "foreign letter" , "Lose pin (anag.)" , "5th letter of the Greek alphabet" , "small number mispronounced, almost completely the wrong way round" , "Greek short E" .)