Letter before epsilon (5)
I believe the answer is:
delta
'letter before epsilon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for delta that I've seen before include "Coastal feature" , "Area of alluvial deposit" , "Mouth of, eg, the Nile" , "Area where a river divides before entering the sea" , "Branching river mouth" .)