Letter from Greece making one's lip quiver (7)
I believe the answer is:
epsilon
'letter from greece' is the definition.
(letter from the Greek alphabet)
'one's lip quiver' is the wordplay.
'quiver' indicates an anagram (I've seen 'quivering' mean this (letters shake or quiver into a new order)).
'ones'+'lip'='oneslip'
'oneslip' anagrammed gives 'EPSILON'.
'making' acts as a link.
(Other definitions for epsilon that I've seen before include "Greek short E" , "Fifth brightest star in a constellation" , "Greek letter - pinoles (anag)" , "Greek letter E" , "Foreign character" .)