Fifth Greek letter (7)
I believe the answer is:
epsilon
'fifth greek letter' 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 "Greek letter - pinoles (anag)" , "Foreign character" , "One's lip (anag)" , "small number mispronounced, almost completely the wrong way round" , "character in Greece" .)